> I would be hesitant to go above 4000IU daily without monitoring blood levels every six months or so.

I did that for a little while (though the nurse practitioner at my work clinic wasn't too keen on signing off on non-medically indicated blood tests). IIRC, I was supplementing at 5000IU a day and after a few months wound up with a higher blood level than I was comfortable with, based on the ranges that came with the tests. After that, I reduced down to 2000IU a day or less.

I've been much less into supplements in general, but I only take 2000IU irregularly now.