If I think of only "よっ" as you say, I come up with various meanings and situations.
"Four" is "よん", not "よっ".
First, "よっ" sounds like "Hey" or "What's up" in English as you say.
In this case, it's a casual greeting used especially among young men friends.
They also say "よう" or "おう" etc.
Second, for example, when you lift a bit heavy thing like a cardboard box, you could say "よっ" or "よいしょ" etc.
In this case, everyone may say so regardless of genders or ages.
Third, when you are about to relax (e.g., to sit a chair or to lie down a couch),
you could say "よっ" or "よいしょ" etc. I personally think that in this situation "よいしょ" could be rather used.
If you JUMP to a couch or a bed, you could rather say "よっ" because it's simple and fast.
Occasionally, "よっ" is used when someone/people is/are about to do something fun/interesting.
The original word for this is "いよっ".
There might be other meanings.
Hope this helps.