Guess it is...I don't know the details, but after checking. Something called Strip Fighter 4 indeed exists. And since Hongfire have it...it must be a valid doujin game.
Yes, it's a real game. It is a japanese game for the japanese PC only. I actually learned of this game after seeing it played at AAC 2010 in New Hampsire. One of the hosts had modded the game to play on a U.S. PC.