Probably not worth getting at this point, honestly. I've played every COD while it was still new and yeah COD4 was awesome when it came out but with everything new that's been added each year, it's just not the same. Even when I go back and play it now, I find myself wishing I had some element of the Create-A-Class that was added in a later game, or a perk that's not even in the game, things like that. It's good that you like COD enough to want to go back and play the old ones, but I'd go back to MW2 instead of COD4. It has more stuff for you to work with, better maps and is actually one of the best in the series.
And mw2 also introduced 15 something kill streaks, that ruined the core focus of all game modes and attracted the scum of the earth to the community
3,5 ,7 with a cap of what can be in the air was probably the most balanced streaks have ever been.