1. The Q4 engine is basically a Lancia Delta Integrale engine (the same source as the running gear) - similar to the Fiat Coupe 16v engine.
2. The Fiat 20v engine, having 5 cylinders, is really too wide for a 146. It is also quite complex electronically/electrically. You can fit an Alfa V6 if you really want to (it's been done to a 145) but it depends how much work you want to do/pay for. If your car isn't a 2 litre then fit a 2 litre twin-spark engine which is an easy upgrade. This engine can be turbocharged to give an easy 220bhp, or tuned more conventionally to give up to 200bhp.
3. If you want to get 300bhp out of a Q4 engine talk to some of the Integrale tuning specialists but think along the lines of re-mapping the ECU, upgrading the wastegate (i.e. increasing the turbo pressure), fitting an intercooler, etc. Plan the whole upgrade in advance, though - don't just throw parts at it because they may not be well matched.