Just finished watching "What If...?" Episode 4. This is probably my favorite episode yet, and certainly had the darkest ending out of all of them so far.
As for feats/scaling, there's quite a bit of stuff, but going off the most notable things I observed:
Doctor Strange Supreme has absorption abilities that he used to consume a variety of different beings, including what appeared to be The Champion of HYDRA. He can channel the abilities and physical traits of those he absorbs this way. A protection spell used by Wong on the other Strange could grant resistance to this absorption, and could also withstand damage from other attacks. Strange Supreme was also able to sense the Watcher's presence. The Ancient One was able to split the timeline so that two Doctor Stranges could exist within the same universe. The Eye of Agamotto/Time Stone can effectively be used for time travel, though "absolute points" in time are nearly impossible to change with this time travel, and Strange Supreme finally gaining the power to do so at the end caused the destruction of the entire universe (he was already established to be causing damage to the fabric of reality with the use of his powers before this) He did seem to be able to briefly slow down the universe collapse, and survived it. It also seemed to be implied that The Watcher could have stopped the universe from collapsing if not for his rule to not interfere.