Hi, I was playing gta iv with mods and I noticed one thing, when I play with left 4 liberty mod there are many more cars and peds on streets. I want to do this without this mod but what file should I edit? Maybe popcycle.dat?

Popcycle has values for max peds and cars for all the different areas at all the different two-hour time spans of the day, starting with Midnight-2AM.

Man can you help me to set up popcycle? Maybe there is some program for it?

IV Needle is a program that will allow you to modify it (along with other dat files). You can also go through by hand and raise the Cars and Peds lines yourself, or just take the popcyle file from a mod like More Liberty.