Actually you CAN emulate the PS2 on the Pi 4, but the speed is abysmal and the legwork required is insane. The tl;dr version of the story is you need to install Windows 10 using Windows on Raspberry and run PCSX2. SOME games run at playable speeds, but not many. Disgaea 1 and 2, Kingdom Hearts 1 and 2, Final Fantasy 10, and Final Fantasy 12 are playable. There are likely more, but those are the ones I've tested.
@vof Nintendo DS, possibly, but definetly not Gamecube or PS2. The latter two are fairly intensive systems to emulate, especially the PS2, and you'll almost certainly be getting very low frames per second. (very possibly <1 FPS)
Those two are really only suited for relatively beefy machines and high-end netbooks.
I have RetroPie running on a Chromebook running GalliumOS and it works very well. As long as the operating system is Debian-based, you should be ok. Updates take a long time because everything recompiles from source, but aside from that it's fantastic.