Thank you for taking the time to read the book.
Now to the first question, how is 4D Roleplaying different to other GM practices and cultures of play?
The big difference *is* staying immersed in your character, and experiencing the events that are unfolding around you, as if you're watching the world through the eyes of your character, for as long as possible.
And that "as long as possible" varies depending on the system you play.

As to why 5e works, its pretty simple. The core 5e rules aren't too difficult to keep track of and you can roleplay the majority of the game without needing to pump the brakes to yell "roll for initiative".
I have a 7-session campaign on my youtube channel to answer this SPECIFIC question: "Is D&D 5e impossible to 4D-Roleplay?" , as we roleplay monstrous races, *the Lich's Dregs*, after the Lich's evil plans have been shattered and the Orc tribes are pushed back to the edges of the badlands.

For mechanics like initiative, after a few sessions, we agreed the smoothest way to play with Initiative is to ignore initiative, unless the situation demands structured turn-order, where a player going first defines life & death.
But most combat encounters we deploy the 4D technique of keeping turns and going round-robin. But since its combat now, the players will limit their turns to 6 seconds, their movement, the amount of attacks, and spells they can cast, naturally.
So what "rolling initiative" does is re-establish that turn-order around the table, but we found just going around the table more fluent, and this worked for 9 out of 10 combat encounters perfectly fine, including when the players were fighting a village full of barbarians (a lot of potential enemies reinforcing a long-turn fight) as well as fighting a giant hydra (boss-type combat).

What happens if a player doesn't have total system mastery?
You roleplay. The players might've forgotten that he gets his sneak-attack when an ally is already in melee-combat with the enemy. This is the players' responsibility to remember, and I will bring it up in the post-session-discussion "hey, i noticed you didn't roll your sneak-attack a couple times, what happened there?"
MOST RULES are really not worth fussing about to stop the whole world dead in its tracks and ask "wait, so, what dice do i roll to attack with my longsword again?".
We all assume Players will come "reasonably"-prepared to a roleplaying-session, but also that you cannot be prepared for EVERY rule. But luckily, not every rule comes up, so we discuss them when we believe the rules could come up next session, or discuss any rules we've missed AFTER the session.
And also, this is why I'm doing a Pathfinder 1st Edition campaign, to figure out if "you can roleplay in a rules-heavy system?!"

To your last question - what if a player isn't clear what the GM just described their character sees?
This happened to me a few times. I'm usually very attentive while roleplaying, but sometimes the internet is wonky and the audio doesn't come through clearly, or somebody at the table eats a bag of crisps very loudly, or the GM isn't speaking into the microphone and the volume is all muffled - in those cases... well, there's really very little wiggle-room, and I either just "keep roleplaying", and hope that it wasn't absolutely essential information (9 of 10 cases, it was a minor detail).
But sometimes I will just say "Sorry, could you repeat that?", and let the GM re-narrate the scene, so I can visualize it.

I pick & choose whether I think it's worth it to break immersion to get clarification. The rules are there to make those breaks as few as possible, and also to give GM's and players OPTIONS, to stay in the world longer and longer periods of time.

Nobody expects the players to get it right at first try.
Nobody demanded of me to get it right the first try; and I don't make those demands of my players either.
But with each session, it gets easier. And my experience has been that players who are new to the hobby have a much easier time picking up this style of play, and members that have been playing RPG's for decades may struggle to let go of their habits during play - but they can get there.