4litres is the correct amount if you are changing the oil filter.
Use a proper VW oil filter - they are £2 or £4, I can't remember which.
ONLY use the 10/40 stuff in your GTi (unless you've done something very special to the engine). I have a 143bhp 8v and I only use the big silver bottled stuff sold at the dealerships. Its cheap and good sh!t.
I used to use poncy Castrol GTX etc which is proper expensive, but not as good - for this particular engine. I noticed the difference in smooooooooothness when I swapped.
15/40 is a bit thick in my opinion, maybe and only maybe would I use it if I had tappet noises or had done mega 200,000+ miles. Although, we do use this oil in our Mk1 GTi.
Change your oil and oil filter every 10,000miles, which should be done anyway when you service it every 10,000miles. (I change my oil about every 5000miles as I'm a bit anal about these things!)