Raspberry Pi 4 can do USB OTG via the USB-C connector (according to this answer) - but only at USB 2.0 speeds since the USB-C connector only connects to USB 2.0 lanes.
My question is - can Pi 4 also do OTG at USB 3.0 speeds? Maybe via the standard Type-A connectors?
No it cannot. No device can do OTG via Type-A sockets, they are host-only.
For the sake of completeness, as @jdonald mentioned in a comment, RPi model A/A+ (the ones without the USB hub, just like Zero) can be configured into device mode and connected to a USB host using a non-standard cable. However, this is excluded for any model B (including RPi 4), as there is a hub in the way.
