Don't know why this didn't occur to me until now, but there was also an Xbox 360 S (the first "good" 360 hardware iteration, arguably the best of the bunch in fact). So an Xbox Series S refresh could be called the S4 (or "S 4", or S^4). With the 360 S being the first "S", then the One S, then the Series S, and then finally the refresh being the fourth S.