There are $9$ unique letters: C, O, N, E, T, R, A, I, S. Of those letters, N has multiplicity $3$ (meaning it appears three times), C, T, and O have multiplicity $2$, and all other letters appear only once.
Here's a lengthy, but elementary approach. Any $4$ letter word you make here has one of the following forms:
1) $4$ unique letters, e.g. CONE
2) A pair of same letters, and $2$ distinct letters (distinct from each other and the pair), e.g. TOTE
3) Two pairs of distinct letters e.g. COCO
4) Three of the same letter, and a fourth distinct letter e.g. NANN
Obviously, none of these types overlap, and no other words exist outside of these types (you have no letters of multiplicity $4$, for example, so no words of one letter), so you just need to calculate how many exist for each type, which is much easier, and then add them together. Try and see if you can do this yourself.
EDIT: I should note that this will give you the same answer Kiran stated, $4436$, if that helps.