That's a non-trivial problem. And the non-triviality in it lies in the definition of "word". What is a word to you? Is "can't" a word? Is it two words? What about "Smith-Jones"? One word? Or two?

Now, suppose you have regexes for words, and strings of non-word characters, it becomes easy:

my $word_pat = qr /.../;
my $non_word_pat = qr /.../;
if (/word1${non_word_pat}(?:${word_pat}${non_word_pat}){0,4}word2/) {
    ... no more than 4 words between word1 and word2 ...
}
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But you have to create the $word_pat and $non_word_pat patterns yourself, as I do not know what you consider a word, and what not.