first are you sure that V4 is downloaded completely? the folder should be 606MB's. right click your V4 folder and double check to make sure.
now you should have placed V4 in the ROR folder that is located in My Documents/Rigs of Rod/packs
once its there all you should have to do is download V4 replica or customs trucks and take the files in those zipped folders and drop them into V4, clear/regen cache and the new trucks should appear.