that's not quite true, they get a 16x up res from their SD mode (some Xbox games supported 720p and 1080i but these modes are not used in Back Compat). that is at best 1920p
some games, for example Panzer Dragoon Orta are below this. Orta runs at 1512p as the original game ran at a very low resolution of 378p
so no og Xbox game actually reaches 2160p (4K) they can reach up to 1920p and some are below this.
they don't get individual resolution boosts, the emulation layer simply does a blanket 16x resolution increase on all of them that means 4x the horizontal resolution and 4x the vertical resolution.
on Xbox 360 games with X enhanced patches this is similar, they get a 6x up res. so x3 on each axis. a 720p Xbox 360 game that has an X patch will therefore render at 2160p (4K)
GTA4 has no patch, it runs at it's original 640p resolution. but it has no FPS lock, therefore on the Series X|S it will basically lock to 60fps. the One X did not quite get there as the CPU was still shit in that one. on One X it hovers at around 40 to 60 fps depending on load.
and yes, 360 games need patches. but they don't add 4K textures or any real change in the game, all they do is run games at higher resolution should they be patched. 3 or 4 games or so also had their HDR capabilities unlocked. again this was not added by Microsoft, this was the games already supporting this but on og hardware the games tonemapped this down to SDR. one such game is Mirror's Edge (which looks glorious at 4K with HDR btw... if they now patched it to run at 60fps too it would be amazing)
og Xbox games are exempt from this as they all get a resolution boost, no matter which game. the boost for og Xbox is 4x on Base Xbox One and S, 16x on One X and Series X and a 9x boost on the Series S
max resolution then (if the original version ran at 480p) for og Xbox games on these systems is:
Xbox One/S = 960p (x4 boost)
Xbox Series S = 1440p (9x boost)
Xbox One X/Series X = 1920p (16x boost)
and here the max resolution for Xbox 360 games (if the original version ran at 720p) that got an enhanced patch:
Xbox One/S = (no change)
Xbox Series S = 1440p (4x boost)
Xbox One X/Series X = 2160p (9x boost)
there is an odd one here tho, and that game is
Perfect Dark HD. That game runs natively at
1080p60 even on Xbox 360...
this game got an Xbox One X enhanced patch back in the day... so by the logic of One X patches this game would get a 4x boost on Series S and a 9x Boost on One X/Series X... this means if they didn't do a special patch just for this game, where the resolution multipliers are changed...
then the game should run at 2160p on Series S and at a whopping 3240p!!! on One X and Series X! that's basically 6k!
I am very surprised that Digital Foundry didn't take a look at this one yet tbh. or
NXGamer
for that matter