Valve adjustment is first simple thing to check and do.

Could be the valves sealing differently in one cylinder than the others. Crap builds up. Valves and seats pit. It happens. You can do a leakdown test of each cylinder to see if there's a difference between them. Only solution for THAT would be to remove the head and have the head rbuilt. But I say just wait until it burns a head gasket and then worry about it.

Other possible causes are ring/bore wear, casting differences between ports and runners, or even cam lobes being worn / off from one another.

I believe Vizard said the A-series was more like Four connected 1-cylinder motors than it was two connected 2-cylinder motors, in terms of how it breathes.

Do the valve adjustment check, reset them as needed, and just drive it.

I’m running a Hiff44.  It’s rather wierd that the one cylinder shows diff readings as I have been told in the past that the A series engine uses Siamese ports in the head so cylinder 1-2 and 3-4 should show same readings?  Maybe i need valve adjustment ?

Could it be burned oil on plugs for 1 & 3? Cylinder #4 usually runs hotter than the others so it runs a bit cleaner.

What engine do you have, twin carb or single? Could be a manifold vacuum leak on that cylinder. Try some spray carb cleaner around that area to see if it changes the idle rpm. Try to do this on a cold engine, it's a bit safer.

Hey guys was going through some maintenance and decided to check plugs.  Funny thing was I found plugs for cylinders 1-3 to be running rich. Plug on cylinder 4 however was running clean almost lean looking!  What gives?  Mini seems to be running Ok.......any suggestions?