The loosing of the four angels that are
bound evidently means they are evil angels.
Good angels are never bound. Some but
not all evil angels are bound.

Peter told us that certain angels that had
sinned and kept not their first estate, have
been cast down to hell. [the word used here
in the Greek for 'hell' isn't Hades which is
the place of the dead or Gehenna which is
the Lake of Fire].

It is "Tartarus" apparently a prison in the
earth possibly associated but not identical
with the Bottomless Pit.

"For if God spared not the angels that sinned,
but cast them down to hell, and delivered
them into chains of darkness, to be reserved
unto judgment. . . ." 2 Peter 2:4