I don't think 400 IU is going to be near enough. Also, you want to make sure it is cholicalciferol (D3) you are taking not D2 (ergocalciferol). Sunshine is the best way, but for many that's not an option. I personally take 4,000 IU per day, and my level is only very slowly rising.
You are right--there is a lot of conflicting information out there!