The four were Carl Schurz, Oswald Ottendorfer, Henry Villard, and Abraham Jacobi. They and Windmuller had much in common. Four of them had been born in Germany and one in Austria, all had been involved in the 1848 Revolutions, and all had migrated to the U.S. They had achieved different levels of education, but all were given the best education their parents could afford and none suffered in later life from effects of an inadequate education.
They were articulate in both speech and writing. They had contacts within the publishing community and a couple of them were publishers or editors as well as authors. Though they were all, in varying degrees, rebels before they emigrated, they had faith in American democracy and, as American citizens, did not advocate force as a means to achieve political reform.
None were prigs. They mixed with compatriots in beer halls and singing clubs, they attended banquets, and were members of clubs where they dined and socialized as well as participating in committees formed to pursue civic improvements.
Four of the five were proud to be political independents. They were Mugwumps and refused to align themselves with a single political party. Windmuller was the only merchant among them, but they all were associated with enterprises, such as banks and insurance companies, that aimed to benefit the public as well as return profits. Most of their efforts, whether commercial, philanthropic, or a combination of the two, began as German-American concerns and evolved into organizations which served all comers.
They all supported improvement in the lot of women and all but Windmuller had wives who became well known in their own right, not as their husbands' adjuncts.
Carl Shurz was an Army general in the U.S. Civil War, an accomplished journalist and editor, a popular politician and the first German-born American elected to the United States Senate. Henry Villard was a journalist who covered battles in the Civil War and later owned and published the New York Evening Post newspaper; he was also a financier and an early president of the Northern Pacific Railway. Schurz was a reporter and editor of Villard's Evening Post for a few years in the 1880s. Abraham Jacobi was a scientist and pioneer of pediatrics, opening the first children's clinic in the United States; he became and remains the only foreign born president of the American Medical Association. Oswald Ottendorfer was editor and publisher of a widely-read German-language daily, the New Yorker Staats-Zeitung. I've written frequently about Windmuller. He was a commission-merchant, both founder and officer of banks, insurance companies, and other businesses, and treasurer of non-profits as diverse as the Reform Club and the Legal Aid Society. He spoke out and wrote about issues that concerned him, particularly corruption in government, national policies affecting the economy, and aid to those in need.
Here's a smattering of articles in which some of the five appear together.
- "With Carl Schurz, Oswald Ottendorfer, Henry Villard and G. H. Schwab [Windmuller] formed, in 1892, the German American Cleveland Union, which proved itself a powerful element in the Presidential Campaign. Mr. Windmuller acted as treasurer, and contributed a pamphlet showing how the prosperity of the country had suffered under the Republican administration. It was published in German and English by the National Democratic Committee, and had a large circulation." -- The University Magazine p. 547.
- AN APPEAL TO GERMAN CITIZENS; STRONG REASONS WHY THEY SHOULD SUPPORT GROVER CLEVELAND, New York Times, August 22, 1892. First paragraph: "The following important address, signed by some of the most prominent German-Americans in the United States, has been issued through the German-American Cleveland Union, and is being circulated among the German voters or the country. Signed: CARL SCHURZ, OSWALD OTTENDORFER, WILLIAM STEINWAY, HENRY VILLARD, LOUIS WINDMULLER, GUSTAV H. SCHWAB."
- The Reform Club: The officers for 1892 were — President, E. Ellery Anderson; Vice-Presidents, Oswald Ottendorfer, Charles S. Fairchild, Carl Schurz, Anson Phelps Stokes, Everett P. Wheeler, George Tucker Harrison, George Foster Peabody, Horace E. Deining, Henry B. B. Stapler; Secretary, Henry de Forest Baldwin; Treasurer, Louis Windmuller. -- Club men of New York: their occupations, and business and home addresses: sketches of each of the organizations: college alumni associations (Republic Press, 1893)
- FOR A GOETHE MONUMENT, article by Louis Windmuller in the Brooklyn Daily Eagle, November 24, 1909: "The time seems opportune for the initiative of a similar movement in New York, where sculptor and poet have numerous friends, who are well known as patrons of art and literature. I refer to men like Jacob H. Schiff, Dr. A. Jacobi, Carl Schurz, Oswald Ottendorfer, and A. P. Fitch. I mention these few on account of their well-known influence and public spirit. The hosts who admire a genius who as poet ranks with Homer, as thinker with Voltaire, as playwright with Shakespeare, are numberless."
- TWENTY-FIVE YEARS OF MARRIED LIFE, New York Times, November 24, 1884: First paragraph: "Mr. and Mrs. Louis Windmuller celebrated their silver wedding last evening at their residence, No. 19 West Forty-sixth-street. Among the guests were Mr. Carl Schurz, Mr. and Mrs. Charles Guidet, Mr. Bernard Roelker, Mr. William A. Hardt, the Misses Hardt..."
Searching the New York Times archive (via ProQuest) gives similar results. In the list that follows the first quantity is the number of hits from the mid-19th century to today and the one following gives hits from 1990 forward.
Schurz, 14,296 all-time hits and 1,028 since 1990The following Ngrams from the Google Books database convey the same general results. The top one shows relative instances in which the names appear in the Ngram data set between 1850 and 2000. The bottom one shows the same from 1950 to 2000.
Villard, 955 all-time hits and 13 since 1990
Ottendorfer 640 all-time hits and 1 since 1990
Jacobi, 360 all-time hits and 0 since 1990
Windmuller, 338, all-time hits and 0 since 1990
Beiträge von Literaten und Künstlern zum Deutschen Hospital Bazaar
Banquet to the Honorable Carl Schurz
Tammany Hall vs. the People's Municipal League by Carl Schurz
Banquet to the Honorable Carl Schurz, Speech of Doctor A. Jacobi
History of German immigration in the United States and successful German-Americans and their descendants, Georg von Skal, 1908
Abraham Jacobi in wikipedia
Dr. Abram Jacobi toast by Carl Schurz, 1900
Geschichte des deutschthums von New York von 1848 bis auf die gegenwart By Theodor Lemke
Schurz, Carl 1829 - 1906 in the Dictionary of Wisconsin History
CARL SCHURZ, PILOT by Mark Twain, in Harper's Weekly, May 26, 1906.
Memoirs of Henry Villard (2 vols., Boston, 1904): Vol. 1, Vol. 2
1911 Encyclopædia Britannica: Villard, Henry
The New International Encyclopædia: Villard, Henry
The Encyclopedia Americana (1920): Villard, Henry
Collier's New Encyclopedia (1921): Villard, Henry
Appletons' Cyclopædia of American Biography/Villard, Henry
Henry Villard Is Dead—Capitalist and promoter expires at his country home, New York Times, November 13, 1900
Valentin Oswald Ottendorfer
The New International Encyclopædia: Ottendorfer, Oswald
The Encyclopedia Americana (1920): Ottendorfer, Oswald
Appletons' Cyclopædia of American Biography: Ottendorfer, Oswald
Appletons' Cyclopædia of American Biography: Jacobi, Abraham
FELLOW-PHYSICIANS HONOR DR. JACOBI, Banquet to Mark Seventieth Anniversary of His Birth
Jacobi, Abraham in Men of 1914: An Accurate Biographical Record of Prominent Men in All Walks of Life Who Have Achieved Success in Their Chosen Vocations in the Various Civil, Industrial, and Commercial Lines of Activity (Chicago, American Publishers' Association, 1915)
Great sound money parade in New York edited by Edward A. Drake (Republic Press, 1897)
The German element in the United States by Albert Bernhardt Faust, Vol. 1 (New York, Houghton Mifflin company, 1909)
The German element in the United States by Albert Bernhardt Faust, Vol. 2 (New York, Houghton Mifflin company, 1909)
 My recent blog post, Secondat: forty-eighters tells of their experiences during the 1848 Revolutions and gives some portraits.
 JSTOR's coverage is more oriented toward high-brow journals than popular magazines. The articles that turn up via its searches are both about the five men and by them. It's important to remember that a search like "Carl Schurz" will not all be about the man and his works; some, for example, concern the park that bears his name. The names of the five are unique to them, so far as I know, so there is likely no "noise" introduced by false hits on other people having their names. The names were not always spelled the same way however. So, for example, the hits on "Abraham Jacobi" do not include hits on the variant, "Abraham Jacoby."
 Note number 2 applies with respect to NYT searches as well as JSTOR ones.
 For information on Ngrams, see Books Ngram Viewer. Culturomics says "The browser is designed to enable you to examine the frequency of words (banana) or phrases ('United States of America') in books over time. You'll be searching through over 5.2 million books: ~4% of all books ever published!"