On 2nd thought, you don't have an electronic MAS like the later cars, but the big air valve in the intake horn does kinda the same thing, so the code 4 may refer to that.
You can do a couple of quick checks to see if the flap valve is functioning:
Pull the air filter:
1. With the engine shut off, the flap should be up near the narrow section of the intake horn, with just a crack around the flap to pass air. Pushing down, there will be a smooth resistance, and the flap will return to the original position. Should be able to slide a dollar bill between the flap and the intake horn, but not much more.
2. With engine running, if you push gently, just a bit, on the flap, the engine should stall a bit, then recover as the system compensates.
These checks will test the basic working of the fuel distriibution and the EHA.

May not be your problem, but maybe an easy place to start.

DG