You are clearly neglecting the whole circumstances of this fight.
Luffy had to learn FS first to hit Katakuri as that was his main issue during the fight. As we know, G4th has a timelimit with very bad drawbacks. He almost got killed by Katakuri when Bound Man ran out, luckily Brulee was there. So you think it would be wise to use a different form that early when Luffy didn't even solve the main issue to hit Katakuri?
We can debate whether FS is an asspull - due to the fact that Luffy learned an advanced form of Haki that fast - but his G4th is clearly not part of that as Luffy used it in the end where he was sure this could turn the tables.
Expect each transformation has legit explanations given the lore of this story.
So what exactly are you criticizing here?
It's clear that his G4th forms are based on the animals he fought in the wild.
I'm expecting another G4th form which will be pure attack based, ie. Tiger/Lion Man.
And yes, this ain't an asspull PU as well.
Now you are just explaining the author's reason for said PU but not the reason why it is an asspull.
Please stay on topic.
Yet again, apples and oranges.
That was in the early plot of this manga when the enemies weren't as strong as now.
Therefore, Luffy has to get stronger to beat those opponents. At the end of TB, Usopp clearly mentioned the constant growth in the enemie's power:
How do you expect Luffy to face Akainu? With no gears at all? Gum Gum Pistol vs Daifunka? Whew...
You started insulting me for no reason and that's why I said it for you to calm down a bit.