The Raspberry Pi4 was released back in June 2019, but it hasn’t yet found its way in to any mass produced handhelds. There are two main reasons for this. The lack of a more compact Compute Module variant of the RPi4 means that any handheld would have to accommodate the comparatively bulky full sized board. Secondly, there is no official build of RetroPie for the Raspberry Pi4 yet. That hasn’t stopped these guys though, and the PiBoy DMG may be the first RPi4 handheld to be mass produced.