In December by chance I put a pack of Vitamin D into my shopping basket. I did not think much, thought to take 1000IE but then decided that for the first week I take 3000 to catch up. Muscle pain went and control over eating improved. I did not expect any changes based on past experience with 1000 but this time I could not ignore it (age can play a role) and I stayed on 3000. Tests a month later showed I was just not deficient any-more. I continued on the regime and started having improvements in long running skin issues to the extent my dentist noticed. It may not be a miracle drug but one should not underestimate cumulative impact individual factors, age and lifestyle changes (less sun) that may change levels and demand.

I used to take 4k IU + K² and think that I'm covered, since 4K units was a lot.

I landed on just above deficient when tested.

My wife was on kind of same regime, but didn't follow it very strictly. She was deficient, but not extremely.

It was quite surprising, because I got warned when buying 4k unit tabs that they were quite strong and pharmacy clerk suggested taking less.

That doesn't undermine OP's point. Being deficient is unhealthy. But that doesn't mean an overabundance makes you healthier.

I am deficient, too, and take supplements (rare liver disease).

I wonder if taking mushrooms soaked in the sun improves absorption compared to supplements?

What exactly changed?

How do you know the changes aren’t placebo?