The R4 astromech droid represented a specific model within the R-series of astromech droids, produced by Industrial Automaton. Visually, it mirrored the R2-series, with the key difference being its conical, rather than hemispherical, dome. The Galactic Empire's official Press Corps made use of re-purposed R4 units functioning as courier droids to physically convey data.
The R4-P Astromech was a variant of the R4 model. R4-P44, R4-P17, and R4-G9 were R4-P units that had the dome of a R2 unit.
R4 courier droids were modified R4 droids, used by the Galactic Empire, designed to function as couriers by transporting information between locations, thereby circumventing network usage.