What is defined as 4MSS? By definition, 4MSS means that something essentially requires more than four moves to do its job. Defensive Eeveelutions are a good example - they'd love to have all of Wish, Protect, Baton Pass for slow switches, Heal Bell and an attacking move, but they can't and have to drop either Baton Pass or Protect (usually Baton Pass), which makes them less effective overall (still useful, but not as useful as they would be otherwise). This is a very good analogy that Steel With It made and this aptly sums it up. Gren doesn't suffer from this.
This statement is actually false. This would only be the case if you are building a team around Gren. However, one rarely ever builds around Gren. If anything, it's the opposite. Greninja uses its offensive option to patch the team's issues.
Support wise, it falls under the characteristic of an Uber worthy mon. If Gren just had Protean and basic coverages, it wouldn't be such an issue. But the thing now is that Gren has not only Protean but flawless coverage coupled with an amazing speed tier. This allows it force it's checks out because you will always be in the "Does he carry this? Does he carry that". The problem with this results in heavy restrictions on team building. You are either forced to pack Scarfers on a more passively oriented team to not be broken down by Gren. Or you are forced to pack Passive mons on a more offensively oriented team to check Gren. And even when you "check" Gren, unless you are born with the ability to read minds, you can't 100% get Gren's moveset. This in itself makes your checks shaky because there is no standard Gren set that Gren will run. They have the flexibility of choice.
There is no reason for Gren to stay in for any of these scenarios. I don't understand why you would bring them up. If you want to use prediction as an argument, it will always work both ways. If he predicts your to spore, he Ice Beams. If he predicts you to seed/mach then, he switches. This is a really invalid statement.
So how is this a Greninja is not ban worthy argument? Blaziken was a fan favorite as well. Let's take a look at where he is now.
This is very true. However, in the case of Greninja, it is different for the sole fact that he has an insane speed tier. The speed tier means that he has more flexibility in his choices whereas the opponent is in a back foot. Let's say you switch in Scizor or Ferro, expecting a Gunk Shot. And you get it right. Does that mean you an stay in and dish something back out at him? No. Because he still has the possibility of running Low Kick or HP Fire. The thing about him is that he has that godly unboosted speed tier that gives him strong pivoting potential and ability. And with that speed, he is backed up by Protean Life Orb boosted coverages. That itself, eases a lot of pressure on the Gren user because he now has the momentum. You see where I am getting at?
Why on earth would anyone let their Gren obviously get statused. The thing about Gren, is that it has the capability of masking what it truly aims to do (like a ninja) and with that insane speed tier, it allows him to be an amazing momentum grabbing mon since the mere sight of him on the opposing team will cause you to play your Gren checks carefully. And once your Gren checks are worn down, Gren's job becomes either much more easier or completed.
Gren is more comparable to Deoxys-S imo. If Hydreigon had 123 base speed, believe me when I say it WILL get the ban.