I replaced a few cars using the Sparc IV and now some of the cars names i replaced are blue in Sparc and also, there is no traffic in Gta IV. Also, i didnt backup the cars...Any ideas how i can fix it?

Hello, If you didn't back them up, you will have to reinstall. I also recommend you to use OpenIV instead of SparkIV. 

Isnt there any way i can fix it? It happened earlier as well but i cant remember how i fixed it

search on google gta 4 backups.

I'd recommend you to reinstall, as downloading files of the internet is always risky. Of course, that is up to you. 

Well, that's REALLY being annoying, you would really go through the reinstall of all the game just for one file??

I didn't say that he had to, I just stated that downloading files of the internet is risky and it is always better to get the files from a trusted source (the disc or Steam). If the OP is using Steam, he/she can verify the game cache instead of reinstalling, this is of course a option. 

I managed to fix it, thanks for the support :)

Glad to hear that :)