You need to take more than 4000 IU/day (probably for quite some time) to have any hope of toxicity. I knew a man who accidentally took 50,000 IU daily for one week (meant to be weekly per his prescription) and suffered no ill effects. Some people may even require doses above 4000 IU/day to get their Vitamin D levels up to target.
I take 10,000 a day. (20,000 is when I start feeling side effects.) It's done wonders for my general mood and energy levels. WARNING: Ramp up and proceed with caution.
P/S/A: There are single sentences which can create life-changing amounts of difference.