Cars are not game specific, EFLC and IV both have separate .img files, the problem with texture mods is most only reach the .06 patch of IV and only betas of EFLC texture mods are out
The easiest thing is to add cars, all you need is two apps, SparkIV and an SDFA dump extractor that gives you a decrypted dump of handling and colour files for cars. If you have the 'non original copy' that is, for an original copy, you need something like xliveless as well and such.
Edit: As long as you don't play online, you dont need to worry about car modding, just need those two apps, ignore previous.
edit: Went a bit car crazy today
https://pic.armedcats.net/b/bl/blayde/2010/07/06/EFLC-2010-07-06-16-01-27-03.jpg
Parked are, from top left, Audi A8 4.2, GMC Yukon, Audi S5, Dacia Duster, next row form top: 288 GTO, G35 Coupe, R34 GTR, California, Chevy Blazer, "The Bandit", Megane 260, 911 Turbo and at the back a 599.