Well, it's a pun more or less.
And I'm not surprised he went with it.
That said, while I agree the names are boring, he wants them to be treated as two games in the same Generation. Both being called 4 helps this effectively. Since Smash is sold on its characters, making sure the rosters stay the same no matter what(which I'm sure he means that even if one version might get a few more, the other one does equally too), it becomes a similar entry for people. The only and only thing that was always changed was the overall roster in each specific game. However, Super Smash Bros. For have the same roster between both, showing they're meant to technically be two versions of the same entry. It does depend which series, and how they're defined as.
I personally like SSBFor cause it's a funnier name. But I can understand why other people like 4. However, the way it's written makes it more clear which is on which system. 4A and 4B wouldn't work. Likewise, 4D and 4W are just as bad. Other than giving them unique names...
Also, he has said later on that he considers them effectively Smash 4 and Smash 5. It's just more based around how different they are besides the roster and gameplay of those characters. It's also probably why Bandai-Namco filed for Smash 6 hirings. To be fair, there are 5 distinct entries now in Smash(one still unreleased) that isn't fan-made. If Smash 6 is a thing, of course it'd have a more proper name like that. I've seen this before; Mario Kart 7 is considered the seventh entry, despite the other games not being numbered. Super Mario Kart, Mario Kart 64, Double Dash!!, Wii, DS, and the GBA version(I can't remember the name off the top of my head). Mortal Kombat 9 did the same thing effectively. And 10, of course. It skipped the number names for some entries by giving them regular physical names much like "Melee" has.
