Mate, the 100 refers to the pitch circle diameter (pcd) which is the circle that runs through all the bolt centres on the wheel.
If you draw a 100mm diameter circle wi a compass, then put 5 equally spaced marks on the circle, they will be 72 degrees apart.
Put 4 equally spaced marks on it, and they will be 90 degrees apart. Surely its obvious they wouldnt fit!
You get 4 to 5 bolt adapter plates but not the other way round. 5 bolt wheels are safer than 4's thats why manufacturers started doing all cars this way. An extra hole and wheel bolt on 5 million cars is a lot of extra cost just on one make of car. VW wouldnt do that for no reason.
Find a set of 5 bolt wheels you like and get them. Forget the 4's they are no use to you, Search for "alloy question or read that post - I put up a list of cars wi their pcd's etc, you can find cool wheels on other cars (scoobies for example) that will fit straight on the mk3. The offset (ET) has to be nearly identical (up to 5mm over or under would be ok but beyond that would wear wheel bearings out prematurely and affect the steering geometry)
