Check your fender drains..

[MPalm]

I replaced the RH front fender due to rust on top of the wheel arch (standard rust on Golf IV, I'll tell why shortly) and discovered that behind the inner lining at the rear of the fender there was a LOT of dirt!
That's where the water should be drained but the dirt prevents this.
I had dirt tightly packed at least 20cm high...

Loosen the lower rear torx-screws on the liner and pry it out a bit and you'll see what I mean.

The issue with rust on the wheel arch is very common and fenders was replaced under warranty.
This is caused by a bit of sound deadening foam placed vertically in the middle of the wheel arch, a perfect place for moist dirt to rub the paint off the inside of the wheel arch.
I think it can be ripped off without removing the fender, just pull out the inner lining and take it out.