Living at 4000 ft seems to be on the cusp of needing different jets to properly tune engines. I have had numerous pieces of equipment that needed the “high altitude” kits to run properly and many others that ran just fine with “normal” jetting. I would like your opinions on wether a basically stock L24 with round top SU carbs would need a different set of jets to achieve optimal tune. Right now I can get the front cab perfect on the colortune, only opened one turn, rear is still a hair rich with the adjustment all the way lean. Compression is solid, ignition timing is good, no obvious leaks etc. Today was my first attempt with the colortune was I was surprised how far I needed to lean it out to get the right color. I was also surprised at how well it ran on my first test drive post adjustment!  More power in the lower range and a lot less fumey during the post drive inspection. 

I would first make sure that you don't have any vacuum leaks anywhere. Any leaks you have will only exacerbate lean running issues you have due to the altitude. Double dog sure.

Second thing I would do is check the needle positions in the suction pistons. They should be installed with the shoulder at the correct location. If someone has messed with the needles in the past and put them back in wrong that could contribute the to uneven nature of your two carbs. Maybe one needle is installed different than the other?

Beyond that... I'm surprised that 4000' altitude is enough to cause that much of an effect, but I can't argue with your results and success.