hi,
it will absolutely take forever (almost...at least it will seem like it...meaning over 6 or 10 or 15 hours...) if you decide to start encryption on a drive that already was set up for Time Machine (even if it was just only set up a moment ago, without any backup so far to it).
as opposed to, however: if you remove a hard disk that has been set up for Time Machine, remover it from being asscociated as a Time Machine drive, making sure to zero it all out, and then, during the process of setting it up as a new Time Machine disk (remembering to pre-select encryption), then it will only take a very few seconds or minutes.
its always been this way.
yr 4TB, if as i mentioned above in the 1st paragraph, will definitely take over 1 day.
give up.
redo as i mentioned in 2nd paragraph.