Hai!
If the fps is low you can lower your setting ofc, but you can also configure your config_mp.cfg to make the game run smoother.
And in cod4 there is a FPS cap, you can unlock it by typing: /com_maxfps 0 in the console or if you want this to be a permanent setting you should type it in your config. It should be located in: C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\players\profiles\URUSERNAME\config_mp.cfg or config.cfg if you mean singleplayer.
You should test out:
/sm_enable 0 disables shadows
/cg_fov 80 field of view
/r_drawdecals 0
/cg_brass 0 disables random flying stuff
/cg_blood 0 disables blood
/snaps 30-40-50 play around with the numbers until it feels good. I play with 30
/fx_marks 0 disables bullet holes, if you are a noob you should not turn this off.
/cl_ maxpackets 30-100 (again play around with the numbers)
/rate 5000-25000 (is said to be forced at 25000 and most likely best
/r_smc_enable 0
Picmip a more advanced way to set quality on some textures. I recomend these values.
/r_picmip 3
/r_picmip_manual 1
/r_picmip_bump 3
/r_picmip_water 1
/r_picmip_spec 3
And if you want a good booster, use gamebooster from IObit. And you should try CCleaner to clean out some of the trash you might have lying on your computer.