It’s a joke based upon the pronunciation of the word, the spelt out pronunciation of the word is as follows: krit-uh-siz-um. Four syllables. Technically there are only three syllables to go with the three vowels in criticism, crit-i-cism, but the joke is of course the spelt out pronunciation version that all the dictionaries et cetera have, in which there appears four, which allows you to argue that there are in fact four.

There are other words where this oddity occurs as well, chasm for example I believe and sarcasm to name a few! Where the spelt out pronunciation versions of the words actually have more syllables than there are vowels in the actual spelling of the word. So, even though technically there are no words with more syllables than vowels, technically there is as well.

Hope this helps! And thanks for reading, Jennifer! Much appreciated :-)