You forget that Goku received a giant power boost before he got beat up and fused with Vegeta. He went from having his 10x Kamehameha tanked by Yi Xing Long to being questionably equal to Super Yi Xing Long (though the latter proved much stronger IIRC), giving him at least a 20x boost from the power he got from the dumb dumb kids.My memory of the SD arc is foggy, but didn't SSJ4 Vegeta fuse with Goku who was already worn down? Goku might take this with Base.
Post.So which Base Goku are we using then? Pre or post having his limits surpassed?
You could actually argue the lower forms got even weaker because after the first shadow dragon Goku literally doesn't use any other form but SSJ4 for the entirety of the Shadow DragonsThe ambiguity of how Super Saiyan 4 works doesn't really lend itself to saying the lower forms got stronger, too, though. It doesn't have a static boost. Sure, we can say it got X times stronger when powered up, but that's not a permanent boost.