£4k, that’s daylight robbery.
Get the car recovered from the main dealer to a trusted Indy. Main dealer labour will be expensive, and depending on the actual turbo, then this company are supplying new stock that Audi would basically fit, whether Garrett, etc. https://turbo-diesel.co.uk/audi-q5-turbos/
Importantly, the oil feed line needs to be renewed, insist on it, as the union nut weeps. In addition, new gasket on exhaust manifold. Oils and filter change as a matter of course.