How to find whether a grammar's language is finite or infinite?
A language is infinite if it can generate infinitely many words. In order to prove that a language generated by a grammar is infinite, you need come up with some infinite list of words generated by the grammar. Proving that the language is finite is slightly more messy&mdash;you need to make a list of all possible derivations, and show that all of the terminate.

In your case, you have the derivation $S\to^*SSSS$, which suggests that the language is infinite. Can you come up with an infinite list of words generated by this grammar?