If you need to match a single word from a range of words you can use regular expression Alternation. This syntax uses the pipe (|) to represent the logical OR for matching. Here is a RegEx that only matches on 1 of the 4 words (depending on case sensitivity):

^(word1|word2|word3|word4)$

If you need to also allow a possitive macth on an empty string you can and another pipe (|) with no word after like so:

^(word1|word2|word3|word4|)$

This will match a blank word or either of the words listed.