HELP.....This car is proper doing my head in.

It has a bad mis-fire on idle only, if you un-plug No.4 injector it doesn't alter a bit.
It run sound once it is off idle with no mis.
We have changed every thing, plugs, leads, coil, we have checked tappet clearances, even swapped 2 injectors over. nothing changes the fault. I've scoped all injectors and both sides of the coil and everything looks fine. All compressions are good and within 10% of each other.

I am now thinking that it might have a sticking valve / needs a decoke but it's a big job for the old lad to pay for if it don't fit the problem.HAS ANYONE HAD ONE LIKE THIS ?