Do you want to spend 4 days in one place, or move along a bit every day?
I'd say your journey breaks into 3 chunks...
Getting out of the south west (up to Bristol)
Getting to the midlands (Birmingham)
Heading north
If you wanted just one stop, I'd say go for the Cotswolds (so between stage 1 and stage 2 of the journey above). Completely different from the coast/Cornwall, and not too much of a deviation. Also very different from Scotland.
You could base yourself somewhere outside one of the towns like Stow, Moreton in Marsh, Broadway etc and visit lots of the area in 4 days. It will be much more pleasant if your 4 days are midweek (all the hotspots are rammed at the weekends!)
Wales is lovely, but the south coast will add a fair bit of journey time, and the north coast you're virtually in Liverpool.
If you wanted more than one stop, maybe do one night in Dorset/Somerset, one somewhere south of Birmingham (maybe close to the Welsh border) and one on the northern side of Birmingham (perhaps deviating towards the Peak District). That's only 3 nights, but perhaps one of them is a 2 night stop.
I'd stick with 4 nights in one place though...