206: The 206 seems to be the better option from your list. You can pick up a 1.4 model (avoid the 1.1, it is slow) with a decent range of equipment though some safety equipment such as ABS weren't standard until mid-2002, avoid the poverty spec LX models, there should be plenty of options on the market given that it is such a popular car. Only thing I never liked about the 206 are the pedals, they are far too close for comfort, could easily step on the accelerator instead of braking (as I've found on my test drive).
Fiesta: You won't get a MK5 Fiesta for that budget so I'd avoid the Fiesta. Rather pointless to get a 5 door hatch if people don't even fit at the back on a MK3/4 Fiesta
Corsa: Most Corsas in that price/age range are pretty poor spec and you'll be looking at Corsa B versions, not Corsa C. SXi and higher spec cars, which is what I recommend (only on Corsa C), will cost at least £4500 and won't even include many features, you'll get a lot more for your money if you choose a 206.
Clio: Plenty to choose from at around the £4000 price mark. The Dynamique model is the better spec model and there should be plenty to choose from at the price/age you are looking for, however, 5 doors are quite hard to find on Dynamique models. Another thing I will point out (again) is the driving position, it is a hit or miss thing. Get your friend to test drive the car.
A car I would have recommended would be the Honda Jazz, but you'll be looking at over the 5k mark to find one of these. Yaris would be a good option if your friend doesn't mind the slow engine, they are well spec'd overall. Plenty of 5 door 1-litre Yaris on the market.
