To unlock, simply complete the current stage and return back and check the category race, you will have additional level 4 and level 5.

Also, there will be a hidden slasher in the previous stage that you have completed, all you have to do is to check your mail after completing level 4 and level5 challenge, and accept his challenge and... get his "valuable parts".

"Black Mitsubishi Extreme EVO 2 and super COLT"
Once you have completed the first 3 season, go to the first stage at FRI night and proceed to Upper hill for Mad Evo 2 with 450ps!(HKS) and extreme powerful COLT at the down hill(325 ps MIVEC).
HOWEVER... you have to obtain the things below..

1)Obtain a 100% win over 150 cars
(note:even 99.9% also cannot)
2)Your Garage mus have at least 6 mitsubishi cars(any type,colt,galant,fto...)
3)The money on hand should be exceeding 10
millions.
4)Tie up wih at least 3 sponsors.(Anyone will
do,enkei and etc)

Once obtain all the above, you can try your luck at every fri night and according to my observation, the two cars came into the parking randomly thus dont blame me if ya couldnt find them.<-perhaps there are other things beside the one i mention inorder to challenge this 2 , but i hope my contribution will be helpful.