On setting up my old Omega II, I had a bit of trouble getting even illumination with a 150mm lens, the proper extension cone, and a 4x5 carrier. The fall off was about one stop at the corners and edges. I finally traced the problem to the lamp position. I believe I remember that the “lamp source” (In my case it was a PH112, a standard size light bulb specially designed and frosted for enlarger use) should be focused on the entrance pupil of the lens with the condensers. (????)
There is still a fall off but it is even and only about a half stop which can easily be compensated by “edge burning” the prints when necessary. The abandoned farmhouse in my gallery in an example of a print from a full-size 4x5 negative with the enlarger set up as discussed above without edge burn.
Check your lamp position.