That's odd!
So when your foot was flat to the floor, the engine rpm's dropped to about 1000?
Did you hear any popping like unburnt fuel making it to the exhaust... how long had the car been running when this happened?
Just to clarify:
If you start from a stop and put your foot flat, the rpm's pick up but when it hits 4k, the revs drop from there, or it gets stuck about 4k?
I had a problem which was in one distributor I got from wreckers where it simply would not accelerate past 4k rpm. It was something in the base of the distributor or the electronics part of it. I swapped the top bits to my other one and they worked ok..
As far as I know, only 1 set of spark plugs should be running when you are at full throttle (or full load) - the difference in timing between firing multiple sets of plugs is too rough at high rpm and would cause the engine to vibrate, and as far as I know the multiple plug banks run entirely based on the throttle position sensor and the temparature sensor - controlled by the ecu. It doesn't sound like that sort of issue to me.
I would say distributor or it could be the computer as you suggest... does it have any error codes?