TM,
There's a whole world of fun available for under £4k. What you generally get is a wooden homebuilt (not kit built) PFA single seater with a Veedub engine.
We're talking VP1s, Luton Minors and maybe FREDs for this kinda money - some are less!.. jolly fun they are too.
Turbs, Jodel D9's and Taylor Mono's go for a bit more.. £4-6K, so the Turb at £4k is quite cheap.
I have a 1/3 share in a FRED. Purchase price was £2.5K so my share came in at the princely sum of £833. For that we got a beautifully built airframe and a virtually new engine but with a cracked crankcase. We have just finished the repairs (which cost £200) so she is ready to go - and people think microlighting is cheap!
You would need some tailwheel time for these types, there will be no POH and control forces will feel very light.
If you have any interest in this kind of flying you really need to be in the PFA. Membership is £40ish but you get a by monthly mag 'Popular Flying' in which nearly all such aircraft are advertised.
Have Fun
Kingy