According the the Oxford Dictionary, mention can be a mass noun, which does not require an a, or a countable noun which does require an a. The Oxford Dictionary states that the countable form is British only, but the American dictionary Merriam-Webster also describes a countable form.
mentioning could be an active participle or a gerund derived from the verb mention: in this context, it is an active participle, which must not be used with an a.
The first three of your examples are therefore correct: the fourth is not correct because an active participle does not take an a.
Note that definitely
is a mid-position adverb, and mid-position adverbs go after the first auxiliary verb- in this case, after would
happening can in principle be used as a singular noun, but it has a 1960's hippie feel. Event would probably be more appropriate, unless you are talking about an event where people smoked lots of hash.
If you apply those two corrections, you get:
That kind of event would definitely have been worthy of mention.