A Ferrari Mondial is a four seater which can be bought for about 15/20 thousand.
If you are really serious about a four seat F.40 you will have to dig deep into your pocket to the tune of about a million pounds. Leaving the engine at the rear you will need a new longer tubular chassis and a new passenger compartment.. The original body is made of kevlar so you will need to design and make a mould. Somebody like Pininfarina in Turin would do the design and build for you, taking about a year so you would have some time to save up. Once you have the car the biggest problem will rear it's ugly head. You will find a four seat F40 impossible to insure because no underwriter will wish to cover the risk of four people being injured or killed. The McLaren F1 has three seats and can be bought for about �650,000.