If its a 2.0 GTi its got the same engine as the Mk3 GTi but with a fancy cover on the top. If its the 1.8 20v, this engine was only in the Mk4 for a short time before being replaced with the 8v!! Read into that what you want, bear in mind that the 8v carries on into the Mk5 and the Beetle whereas the 1.8 20v normally aspirated has died a death AFAIK.
I just sold a Seat Toledo V5 which is built on the same floorpan as the Golf/Bora and the build quality, ride quality etc were a massive leap over the MK3.
Yes, I have had a lot of cars, 40 to be precise...