What Does "P4" Mean? (Rebus)

[Bert45]

In an episode of the current Rebus story, Rebus's brother, Michael, was robbing Darryl Christie (I think). he and a couple of his mates were in a large dark warehouse. Suddenly, somebody appeared with a gun, asking who the F they were. Michael shot him (through the head) and said, "He's P4." Probably army slang. What does P4 mean? Thanks.

[elliemay1]

//Priority 4 (P4) – The issue is an inconvenience or annoying but there are clear workarounds or alternates//

[Bert45]

Oooh! That was quick! But, if you know the answer, you don't have to waste time googling it. Thanks, elliemay1.

[Bert45]

Why don't they put this sort of information on the screen, like subtitles? – Just to annoy people like me.