It all comes down to the software. While the Shield has CUDA and a “real” Maxwell GPU it’s much older than the Pi4. However having said that the Pi4 GPU is completely new and developed specifically for their board. Rasbian uses a new rendering engine and all, with potentially a little more compute headroom.

As of right now they are at parity in my experience, both can do Dreamcast and prior except N64. Which both struggle with, but I’d say the Pi4 has a better chance of catching up. They both do PSP poorly as well. PS2 and beyond is simply not possible on this tier of hardware.