OK, as people have said £4k will buy you a cheap S2 and it will very likely be cheap for a reason, though you might be lucky. As has been said on here many times before the cams and chain are the S2's Achilles heel in that they may be in need of replacement and will cost well into 4 figures if they do. The tensioner isn't too expensive if that's all you need and the chain alone is not frightening, but I think it's a false economy to replace a worn chain without replacing the sprockets it runs on. Search and you will find lots of buying guide info on all models.
You're going to struggle to get a late Turbo in any sort of shape for £4k as well, but if you do it will be a better starting point
in my opinion. The reason I say that is that on track the revs are never low enough to be beneath the range the turbo is blowing at and you get bigger brakes, LSD, stronger drive shafts and gears etc. etc. Turbo's unique problems are with the wastegate which will be weak or maybe even cracked, but being 8 valve they have no timing chain. As a plus you can replace it for not too much money and get more power. Turbos are very tunable whereas the NA enignes are much more limited. Another way of saying that is it can be much more expensive to have a Turbo engine than an NA of course.
If £4k is the purchase budget and you then have preparation budget then you might have a shout, especially if you consider fixing any minor niggles as preparation. I'd say it's best to assume £1k to sort a car out when you buy it in any case even for the road. For preparation consider this: I have spent (parts only) around £2,300 on suspension, £910 on wheels and tyres (that's 1 set), £1,700 on brakes (could be done cheaper I guess), £600 on seats, £250 on harnesses etc. Including a full rebuild of my late Turbo's engine I reckon parts alone total £17k+ with maybe half of that being engine rebuild and upgrade parts. I have probably recovered something around £2k in parts I have removed with loads more in the garage to sell still.
Clearly you don't need to do all that but not much is cheap for Porsches and the sort of S2 or Turbo you could get for £4k is very unlikely to be much fun to drive on track as bought. If you find a box-fresh one it might be OK, but then again if you have a dedicated track-prepared Golf then even a brand new 944 would feel soft in comparison, so maybe you
will need to do the coilovers/bushes/Rose joints/ARBs and even an S2's standard brakes are probably borderline for track; those on a lesser model (S or Lux) certainly would benefit from an upgrade.
Overall they make a good track car but they don't make a cheap track car. If you really have £4k to spend then what about a 924? You could get a race-prepared one from the Lodge championship for your budget.