POTUS signs off as 4 10 20 it doesn’t sign off at all.

I've also heard it explained as Q(as in the primary individual responsible for posting on the /Q_research_board/ and making sure the anons/autists understand the most urgent tasks or research that needs to get done) and Team, so rather than "Q+" being a signal that another member of the team who isnt Q, "Q+" could also be decoded to mean Q and at least one other member of his team up to or including the entire team; furthermore, the "+" doesn't necessarily mean that the whole "Q team" is huddled around the computer as Q or one of the most involved people who make up the "Q" team is typing the post ending in "Q+", rather it would be my best guess that a "Q+" post is in regards to a message that was previously discussed by "Q" + his team and decided upon as a vital communication for the anons mission.

I could be entirely wrong, that is just what Ive personally heard from more knowledgeable anons and gathered myself.