That happened also in 4.6 and it's not specific to RetroPie.
Since the Pi4 supports 4k, by default it will use the video mode advertised as 'preferred' by the TV. If that's 4k, then this is what it will use. Previous models didn't have this issue, since they didn't support 4k. You can check with tvservice -m CEA what video modes your TV supports and what is the preferred mode.