What does P4 mean in practical terms?

If whatever is being photographed has change or motion in it,

. . . then the surest way to make a TTG photo of it is with a single uninterrupted exposure

(and even then the length of the exposure may be limited by the sasibe standard).

Conversely, if the photographer wants to combine exposures,

. . . then the surest way to make a TTG photograph is to have no change or motion present in the scene.

See FAQ #14 for more