I'd heard 2x your car's weight, in Lb a 4,000lb vehicle needs 8,000lb winching.
My local specialist now does a 8,500lb winch with 5.5hp motor for under £400, look on E-Bay or can give you a number.
The rating doesn't always match the power of the winch though, a 9,000lb winch might have a 3.5Hp motor so is less capable, or bloody slow. So I'm told anyway, until funds allow I'm still on the manual winch.