It doesn't matter if
Grand Theft Auto V is coming out.
Grand Theft Auto IV is also certainly worth playing. Besides, the usual price for it isn't that high anymore, so it's not as if you'd be paying sixty dollars for
Grand Theft Auto IV.
Also, there's such thing as
Episodes from Liberty City, which is the pair of DLC games that came out as part of the IV era. You might want to look into that, too.
I hope this helps!