I recently got a Toplight Flat S
Busch & Müller: TOPLIGHT Flat S and have been very impressed with it. It's fitted to a Tubus Cargo, and it has the option of 50mm or 80mm mounting, you just push out the plastic from the back with a screwdriver to make the relevant hole for fitting.
The 4D is a fender mounted light, so not applicable if rack mounting.
For power, neither of these lights are USB, I haven't seen a B&M rear USB, for power I use Energizer Lithium AA's which last far longer than regular (Alkaline) AA batteries, and are very light weight as well, not that it matters.
For blinking vs steady, would prefer steady if only one light is fitted, if I am going out after dark, I fit an additional Smart (Planet Bike) light as well which can be used in flashing mode.