Yeah they should fit mate, mk4 sidelights are 501 capless bulbs.
I wouldn't fit those though, the cree bulbs are really bright and they get quite hot too, and you might get pulled for them (possibly if they see you coming head on).
Technically sidelights aren't suppose to be super bright, just sufficient enough for the car to be seen (I believe).
What you should look for are the 501 bulbs with LEDs around the sides as well as the front, this will light up the reflector better in your headlight.
I had some 'dabhees' 501 bulbs in my a3 and they were quite bright and did the job perfectly.
Linkage -
http://dabhees.co.uk/canbus-side-light-501-w5w-t10-168-194-bulbs-x-2-115-p.asp
