Jay Jaffe has just informed me that the Niekros have been confirmed. That's #5:
Found this for Joe Niekro's entry at BaseballLibrary.com (http://www.pubdim.net/baseballlibrary/ballplayers/N/Niekro_Joe.stm):
"He was 5-4 head-to-head against Phil and homered off him in 1976." And down below, in the chronology section: "May 29, 1976: The only home run of Joe Niekro's 22-year career comes at the expense of brother Phil Niekro as the Astros tie the Braves in the 7th, then win 4-3. "
Checked similar pages for the brothers Perry, Forsch, Perez, and Dean, but found no mention of any of them facing each other. The search continues...