Isn't SpyXFamily on its way to outsell per volume everything in WSJ bar One Piece.
SPYxFAMILY was the second fastest-selling title in all of Shueisha only behind Assassination Classroom, and having it now 4 million copies for 4 volumes we know it has surpassed already the pace AC had, given this one had 3.6 million copies (by Volume Obis) in circulation by the time Volume 4 went on sale. Of course, we know it's not an exact 1 million per volume for SPYxFAMILY because of the data we're aware of, which is more than a million for the first 3 volumes and a little less for the 4th one. So we have 3 options:
One is to actually say that SPYxFAMILY is ahead of every single current Weekly Shonen Jump title compared to when those had 4 volumes out (with maybe the exception of ONE PIECE given we don't have exact clear data on its copies by Volume 4 but doesn't show indications of it being 4 million, although it did have 6 million copies by the time it reached Volume 6):
Haikyu!! had 1 million copies, Hunter x Hunter had 3.7 million, My Hero Academia had 1.5 million, The Promised Neverland had 1.1 million, Black Clover had less than 1.6 million (number given by Volume 7), and the rest we know about so no need for me to list them since they're all currently below that.
The second one is to keep in mind that Volume 4 of SPYxFAMILY is not exactly 1 million copies and not say that its volume/copies ratio is ahead of every Jump series since it's not 1:1, but quite a good amount of them. We already know its print is at the level of some series such as Haikyu!!, The Promised Neverland, My Hero Academia or One Punch Man.
The third one is to ignore this fact, consider that every volume equals to a million copies, and confirm its ahead of every single series in Jump right now in volume/copies with the exception of ONE PIECE and Hunter x Hunter. I would count The Promised Neverland or Kimetsu no Yaiba here but they're not running anymore.