I'll proceed by reverse probability, that is, I'll find the probability of having the sum of the two cards less than 4.
When does this happen? It's when you get 2 aces (sum = 2) or an ace and a two (sum = 3)
No. of possibilities for 2 Aces = 4C2 = 6
No. of possibilities for 1 Ace and a 2 = 4C1 x 4C1 = 16
Total = 22.
Now, total number of possibilities = 15C2 = 1326
Hence, probability of having a sum of 4 or more (at least 4) is given by:
2. (EDIT: Removed as question was misread, sorry)
I hope it helps!