Hi @Marruger,
An RPi3 is sufficient as the onboard computer for running an ROV with a Pixhawk and a camera, but if you’re doing additional processing (e.g for additional component integration like custom sensors and actuators, or running computer vision and/or custom control algorithms) the additional processing capabilities of a better board can be helpful. RPi3 also only has USB 2 ports, whereas an RPi4 has two USB 3 ports which may help with integrating more powerful / higher data-rate sensors, and also has a faster ethernet port (although communication is still limited to <100Mbps if using Fathom-Xs and/or our Ethernet Switch).
The additional power on an RPi4 can also be used for supporting things like our upcoming flight controller, which will replace the Pixhawk in new systems.
Hope that helps! 