There were officially 20 Q4s sold in the UK, all of which were special orders - in other words they didn't keep them in stock.
All Q4s are lhd, and all 20 UK cars were the early 'narrow bodied' examples.
Cars were imported from Europe, so there are obviously more than 20 in the UK and there are even a handful of later wide bodied cars, 3 that I know of for sure.
I had a French import early car which had be modified to about 240bhp. It was awesome, but I couldn't really live with lhd. If you have an Integrale then this shouldn't be a problem.
In my experience, Q4s are more modern and better built than Integrales, but not a raw or exciting as an Integrale.
A good person to speak to is Barry Waterhouse at EvoCars (evocars.com) as he has vast experience of both Integrales and Alfa Q4s.
If you're looking for one in Europe a good source is mobile.de for cars located in Germany. This is the best place to find a later wide bodied car.
Hope this helps.