HW4 itself has nothing to do with Steam. I mention it, because Model X cars with HW4 also have a different MCU (still unnamed?) in comparison with previous cars (e.g. MCUZ). Unfortunately so far I haven't seen confirmation about what's inside Model Y cars with HW4...I highly doubt it will run Steam just because it has HW4. It'd be announced in the release notes of a software update as it'd have to be added to the 3/Y code.
From what I read - to run Steam you need the discrete Radeon GPU.
- MCUZ in Model S/X - has daughterboard with discrete Radeon GPU
- MCUZ in Model 3/Y - doesn't have the daughterboard (except for some of the first units manufactured). Can that be purchased and installed as upgrade?
- MCU coming along with HW4 in Model S/X - has discrete Radeon GPU chip soldered on the main MCU board
- MCU coming along with HW4 in Model 3/Y - ???