Captain84000 has only appeared once, in Starman3's visit to the Fourth Dimension. He was revealed to be a member of 4DR.
He hasn't appeared since, and probably never will, because of Captain's discontinuation of his and all counterparts color codes.
Counterparts and Other Relevant Characters
Mushroom Kingdom/Blooper Land counterpart: TheCaptain64000
Star World counterpart: TheStartain64000
Dimension X counterpart (doppelganger): ThePurpleWolf666