4000 IUs is the amount of vitamin D your body needs to function well. Vitamin D can come from food sources as well as the sun so you'd rarely if ever, except some people in deepest winter, need 4000 IUs. I was on 4,000 in late December but wasn't on it for long.

The vitamin D needs of people are very individual and can vary widely due to latitude, ethnicity, heritage, and gender. Thus people should really get their D tested and supplement to achieve optimum levels.

However, that is why I suggested starting at 400 IU and working your way up. The 400 IU is the most common per pill/gel dose. We have conducted an experiment on vitamin D and posted in the Supplement forum. It's from that that we learned the wide range of requirements and reactions. We also found that the vitamin D in a multi was considered useless.