Make sure part4.rar is roughly the same size as the other parts (except usually the last one). A smaller file size might indicate an incomplete download.
The Koenigsegg Regera holds the highest top speed in the game at 245 mph.
Split archives (like .part1.rar , .part2.rar , etc.) are fragments of a single large file. To use them, you generally need (Part 1 through however many there are) in the same folder.
You can unlock "Ultimate" performance parts by winning High Heat events and successfully returning to a safe house without being busted by the police.