The range that is considered safe is much smaller than the range that is actually safe. It also doesn't help that there is misinformation about what "vitamin D toxicity" actually is.

Wikipedia states "Vitamin D toxicity, or hypervitaminosis D is the toxic state of an excess of vitamin D. The normal range for blood concentration is 20 to 50 nanograms per milliliter (ng/mL). However, the toxic state is known to be a value of 100 ng/ml or more in a clinical setting."

This is wrong because toxicity is not a pure function of vitamin D concentration. I'm above 100 ng/ml and yet my calcium is normal. The toxicity is purely a result of elevated calcium levels. The threshold level of D which causes this elevated level of calcium seems to vary significantly between individuals, and in any case, is much higher than the numbers that google or wikipedia gives you. At any rate the 4000 IU number is unscientific nonsense.