Man, I'd hate for you to miss out on such a fun game..
I can see why you'd see them as similar but gameplay wise R6 and CoD4 are pretty different. 
Call of Duty could be summed up by saying it's a semi-tactical fps, as in bullets hurt and depending on the caliber can go through a wall(s) and into your skull.   BUT on the flip side of things the gameplay, whether it be free for all, team-deathmatch, capture and defend, or assault (think CS, planting the bomb-ish type of match) the action feels fast paced and exciting.
I'd encourage you to check out the demos available because I've played both and even though i like the ideas of vehicles both manned and unmanned in Frontlines, the gameplay felt typical, detached and in bad spots down right awkward.

I usually check out gamespy for reviews mostly because they're well written and even though the scores can sometimes be swayed by publishers I'm sure, the content they put up with the review  (in pictures, movies, links to demos, etc) helps get a better feel for the game.

Another place I usually check out for info on recent releases and upcoming games is Destructoid.  The community there is pretty crass and very off the deep end but it's raw, and unaffected by EA's pr department :P

Anyways, here's a link to GameSpy's review of both games.
Frontlines

Call of Duty

i have cod4 and I like it. For some reason i keep playing it so something is good about it.

If you like team-oriented play (for the most part except for FFA) Cod4 is a great game. There of course will be the down-right 'noobs' with nade launchers and whatnot, but for the most part, its fun and can be intense at times.

The main things I dont like about cod4 is the ranking system, its too fast to get the highest rank, and that the maps are generally small. But besides that, its a great game.