Ah, you're probably right, Jim! Senfter's Wikipedia article lists it as being in B major, but klassika.info and Shamus' post say B-flat major. Aw, and I was really hoping for a B major symphony ;D! The only other B major symphonies I am aware of are Shostakovich 2, Haydn 46, Korngold Sinfonietta (a symphony in all but name), and a symphony by Georg Mann!
The beginning of J. Senfter's 4th is tonally quite ambiguous, rather in a Franz Schmidt vein, but then it centres around B-flat indeed - and the conclusion is unambiguously in B-flat Major.
Here are some B Major symphonies: Philip Greeley Clapp's Sixth "Golden Gate" and Tenth, Robert Farnon's Second "Ottawa", Edward Burlingame Hill's First, Feliks Łabuński's Symphony in three parts, George Lloyd's Fourth "Arctic", Cipriani Potter's Third, Charles Tournemire's Second "Ouessant" (starting in B-flat minor, but ending in B major), Andria Balanchivadze's First, Alberto Williams's Sixth "La muerte del cometa".