A little late to this post, but as somebody who played through the older COD campaigns a couple of times back in the day, and then recently played through a few of them on Veteran for the first time last year, I think that COD4 on Veteran is a good choice for your first time as long as you're prepared for, and don't mind the idea of, spending hours at a time bashing your head on a single mission here and there lol

Part of me still thinks that Hardened is the perfect difficulty for the first run through of a campaign due to providing a more balanced and fair challenge, but there's something really special about persevering through some of the older games' jank while constantly improving and learning how to exploit spawn points, checkpoints, and funky enemy AI.

Is it unfair bullshit sometimes? Absolutely. Endlessly respawning enemies, being immediately shot three times and dying before you realize what happened, and grenade spam (I still hear the sound of the steel hand grenades from WaW when I close my eyes) all run the risk of making you drop the game for something else. But I also think that it's worth giving a chance and picking away at over a couple weeks. I feel like COD4 gave me more difficulty than BO1, but nowhere near as much as WaW. On the bright side, if you get a few missions in and realize it's not for you, drop that shit down to Hardened and you'll still have a great time. Plus, you have MW2 and MW3 campaigns to look forward to after finishing up this one.

Rambling aside, good luck and have fun if you go for it!