Re: Loss of power at 4000RPM.
Had a look for the lambda heater fuse. On earlier models it appears to be fuse B36 - 7.5A on the main block. On later models with multi lambdas it seems to be F2, 15A (one of four on a separate auxiliary block, B98) ? Best I can do, you will see how vague the CD is if you get one.
Doesn't actually sound like a lambda problem to me as lambdas just make small changes to the mix up to about 3000 rpm to pass the MOT emmissions test. If your happy with the ignition then fuel might be the next thing to try - the fuel rail and injectors come out quite easily.
What's the cam belt history ?