Topic: cb550 '76 running very hot on number 4, and cold on number 1

[kippstakes]

I recently acquired a infrared thermometer and decided to do some dx on my cb550's pipes to see how evenly she was heating up.
Cyl 1 was running cool, which I had suspected (still trying to solve this little puzzle- it seems to be getting gas and spark, and has good compression relative to 2,3,and 4).

What I was surprised to find was that cyl 4 was running extremely hot. I was getting about 180 on 1, 300 on 2/3, and over 475 on 4. What the?  Oh and I should say that the bike was warmed up standing (not ridden), and the measurements were taken at the first pipe elbow, just in front of the head.
Any ideas what would cause a single cyl to run relatively hot? I'm vexed.
Thanks

[flybox1]

carbs recently vacuum sync'd?
plug color? (rich or lean)
bowl fuel level correct?

[kippstakes]

Fuel level should be spot on- I did a clear tube test on all four about 9 months ago. The carbs were vac synced at about the same time. I've tried rotating plugs just in case, but the symptoms remain.
I haven't done a plug chop in a while though, that will be the next step.