04:58 in Scott Manley's new video NASA Might Be Giving Away A Saturn I Rocket - Here's Why I Love This Vintage Booster he says:
So the four engines in the middle of this thrust plate (plane?) on the bottom are fixed; they just hold position. The four on the outside they gimbal, and they provide all the roll, yaw, and the pitch control.
There’s another, there’s an interesting difference in these engines as well. The four engines in the middle, they exhaust their gas generator exhaust over the side; there’s these little flares coming out the side.
But the outside ones, they use a gas generator exhaust around the tip of the nozzle, so it looks like a film cooling… dark, and then you’ve got the light coming through here.
I think that’s great, and I never quite figured out why this decision was taken, and I suspect it’s got something to do with the engines gimbaling. They didn’t want the super-hot exhaust to impinge on anything.
Question: I don't understand what the challenge would be to point the gas generator exhaust from the four outer engines outward and away from the rocket so that even when they gimbaled it wouldn't hit something. Why would this have been so difficult? How could this be a proposed explanation for this 4 vs 4 configuration?
I've added some screen shots and a few arrow annotations of things referenced in the block quotes.