4. We aren't going to be able to take enough tricks in notrump. If your partner has something like KJ76432 without an outside ace, you will probably make 4 (on a lucky day), but would not be able to make 3NT because you won't be able to get to partner's hand. Below is an example of two hands: 

You selected Option true.

KJ109432
Q65
3
87

You selected Option true.

A
KJ42
AKQ84
KQ4

We can see that 3NT will need some help, but 4 is virtually cold.