I have never attempted to do this, but having modded the weapon sounds I would suggest you locate the file containing the vehicle sounds. Open and extract the file you wish to replace, look at the format of the file (.wav, .ogg, etc...) and the sample rate (generally measured in KHz if memory serves me.) then grab the San Andreas file you wish to use and compare the two. If they are not already a match then download a freeware program called Audacity
You should be able to import the San Andreas file and export it as whatever format you need for the file to work in GTA 4.
The above information assumes I know what I am talking about.
I may have a couple details wrong, but if you do the above you should at least be moving in the right direction.