This especially if running the barrel it was blue hard edge all around but in other focuses cannot get back to there leads to say a
lens train problem - perhaps a lens not properly seated or something but below the steps in troubleshooting I would take.
I don’t have a tremendous amount of experience with S-4 fixtures, can almost count on one hand how many times I have bench focused a group of them at any sitting. Had a few examples of them which no matter what I did, I simply was not able to give the fixture a proper bench focus that I would hope in problem was later solved by those that more normally do so.
Think it’s in part a question of as opposed to say a 360Q
Leko that bench focuses in three dimensions, the S-4 only does so in two dimensions. Make sure your lamp is properly seated and hope for the best as a limitation in easier focus but in some ways three dimensional three screw focusing while a pain in the rear has its advantages.
On the other hand at one point I got to service a Lycian 1290 follow spot that someone forgot to strap into the man-lift lifting it up to a lighting position. It fell and in getting back to the shop I gave it my best effort in optically re-aligning it. Got it back to a hard edge and even reduced the double image but end result was that I could not get rid of the half brown/half blue hard edge with a slight halo I expect you in part also see. For me it was the slightly bent frame and
reflector warp caused by the fall that required a few $K in shipping and service at the factory to correct.
Overall, since a S-4 reflector is less able to go out of round and works or won’t I would suspect it’s more about the optical train being straight. This either fully seated lamp - and or properly seated lamp and a frame for that lamp that’s not warped given a cast element or lamp cap fully installed, or perhaps a lens train that’s optically parallel in the lens train.
A few things to be difficult here. First and easiest in troubleshooting is knobs tightened. Second in changing lens trains, third seated lamp. After that and with all bench focus of the lamp in all conditions being adjusted.
Knobs tightened or stripped could make it seem like there but not in a bit of wiggle room. This as with missing lens train glides could allow for a slight out of focus cant. Lenses in the lens train by lot number or ones not fully or properly seated could be at a slight angle, bumpers for them missing
etc. Seated lamp being a lamp and fixture question. Hard to get a lamp fully seated and in doing so enough slop in being able to do so that it does so properly. Could be wear, could be bad casting on the lamp or fixture, could be any number of reasons a lamp don't seat properly in a pre-focus socket type this fixture has. Most commonly it's a lamp not fully or properly seated however, or perhaps a bent casting in seating it as less common.
After that, I would suspect the optics of the lamp socket alignment and change a lamp cap with one that is known to work properly in bench focus. Following this troubleshooting concept of it being the lamp base I would if still bad suspect that if I changed cap and barrel, a slight warp to the fixture. I would refine that search in changing out parts until it was corrected.
Should be able to correct this problem, just take a bit of time. And somewhere between all also play with a different lamp in the fixture.