Wesley Snipes doesn't look too bad for his age... He probably could do a Blade 4... it would be interesting to see who would be in it... and if it could lead to some cool "Easter Eggs" spawning possible cameos from other cool Marvel Characters.
If Stallone can still do action movies, then Snipes can too. But honestly, Blade 3 was kinda lame (and 2 too, in my opinion), so I'm not sure if I'd like to see a Blade 4. Though honestly a PG13 Blade movie would be the saddest thing ever.