Yes, thanks for adding this. You are correct that I was aware of that as I discovered it in my hours of research deep-diving multiple govt agency published recommendations and the original peer-reviewed papers and meta-studies they are based on.
I should have included it as it's something people should know. As you likely already know, there is a lot of excellent research on this topic and, as is often found, there is a range of recommendations within a broader consensus of credible expert groups. Different govt agencies and relevant professional medical associations have released official guidelines in different nations and timeframes. Some find the 4k IU upper threshold may be somewhat too low based on more recent research data (a few, mostly older ones, are even lower than 4k).
All generally agree adverse consequences are rare and that it's not usually possible to get "too much" D from sunlight alone since it's naturally regulated in otherwise healthy people (but other non-D potential negatives from excess sun exposure still apply). Overdoing dietary supplementation is still possible since D can build up over time in the body if sustained supplementation dosing dramatically exceeds an individual's needs and natural clearing rate (which can vary based on age, mass, health condition, gender, complexion, ethnicity, some medications, etc). There are also a small number of individuals who seem to be unusually sensitive to sustained excess D supplementation but this also appears to be rare.
As always any substance, even H2O, can cause adverse reactions if consumed in substantial excess of a given individual's tolerance and clearing rate. Given the primary audience, I was assuming (perhaps naively), HN readers were more likely to actually do the necessary due diligence to verify and appropriately adapt any generalized advice for their context.
Regarding D3 dietary supplmentation in light of current CV19 concerns, I suggest everyone considering it as part of their risk mitigation approach still balance the likely benefits against the possible risks on an individual basis. IMHO, supplementing an otherwise healthy child would be unnecessary and likely inappropriate unless medically advised.
My mother is in her 90s, overweight, pre-diabetic and walks only with a cane. So, based on her weight, age, gender, conditions and elevated risk of serious harm from CV19, I suggested starting on a somewhat higher D3 dose and we've gradually tapered it down over months to avoid any excess buildup. As an additional precaution, I've also had her supplementing K2 because sustained excess D3 supplementation can effect the relative balance of these for her. I also took photos of the labels of her current supplementation (calcium, multi-vitamin, etc) and checked the literature for the combined total amounts for adverse potential interactions. All while keeping her gerontologist apprised at every step via email on what she's taking and our reasoning. To ensure she's taking the supplements correctly I check in with her specifically about the supplements every week. I also have taken over buying the supplements and have automatic subscriptions being delivered at the appropriate frequency as well as getting her a daily pill minder to help ensure she doesn't forget and miss a dose or double dose by accident. I'm also having a variety of healthier prepared hot meals that she enjoys delivered to her on a regular schedule to assist her in weight loss and sustaining a more balanced diet.
Hopefully, everyone exercises similar diligence as good health is only partly in our control, so we shouldn't leave that which can so easily be managed to chance. However, even positive steps toward good health must be balanced in light of individually unique risks, preferences, priorities and life satisfaction. There are some things she could be doing to be healthier but sometimes she makes a conscious choice to not adopt them. Simply being alive isn't always the same as living as happily as possible. It's her life and she should live it as she chooses (self-autonomy can be vital to happiness too). I try not to chide her on health choices but rather to ensure she's able to make fully informed choices (which sometimes includes the choice to not choose). There's a fine line between caring concern and being overbearing. I never want to slip into implying she bears a duty to live her life (or stay alive) for the benefit of those who love her. That means supporting her freedom to fully enjoy the few 'guilty little pleasures' that she chooses - especially in the twilight of a life so well-lived. Anything less wouldn't be returning the unconditional love she has given us.