I'm on 575 bhp at the fly on standard ECU and 1000cc injectors.

Nothing wrong with standard management if you go to the right people

For £1k drive in drive out, you can get a Rod job off Norris Designs - good for up to 550 bhp on standard pistons.

Decent turbo like an FP Green on S205 or even a RED and you're knocking on 450-550 bhp as long as supporting mods are covered (decat, pump, 3" exhaust and elbow, plugs air filter etc)

Add a decent clutch (£1000+ for a twin plate) bigger injectors (£300 new) and a few odds and sods and I reckon you'll have maxed your £4k budget, but have a good working 450+ bhp

Best to get a car with some mods on it already, buying all parts brand new they are pricey. Buying 2nd hand is far cheaper when you know what you're looking for and at!

Si