Ah jeez.....I can't remember off the top of my head. I know it's more than 40 atleast.
Edit: I just did the math in my head:
42 in UN3 - 2 (ANBU Kakashi and Might Suit Naruto) + 6 TS Characters (TS Naruto, TS Sakura, TS TenTen, TS SHikamaru, TS Neji, TS Lee) + 2 new Akatsuki (Deidara and Sasori) + 1 other character (Granny Chiyo) = 49.
I could be wrong and maybe missing a character but I am sure that there are 49 characters in UN4.