Well a person can take too much BUT you are no where near the amount it would take to do that.
I was not sure if you were saying 4000iu total or add 2000iu to that plus what is in the calcium? We usually recommend an average weight person of 150lbs take 5000iu a day OR the amount it takes to reach a level of 50ng/ml (125nmol/l). If you have any health issues, then more or a higher level might be recommended.
Vitamin D helps to absorb calcium from your intestines. So once people take a healthy amount of Vitamin D, they may not need to supplement with a high level of calcium.