The quests don't really require you to be at a certain level, they just recommend a certain level so you don't get your ass easily whomped due to being greatly overpowered. Having said that, I've done some level 7 and 9 quests at level 4-5 and completed them although it was challenging. A good story SHOULD have some level of sequence to it though too, you don't want things so wide open and non-linear that you can walk right from the start of the game to the final battle or whatever and defeat the boss monster at the end without playing the rest of the game. When a story is being told and unfolding, you have a natural linearity to that, but in between each element you have non-linearity.

That wasn't what was said. A good developer who wants to make a good open-world game (Well big zones,) will manage to add linearity to the game through different means. Like design it so that you would stay in areas that you can handle (In this case, make those areas huge.), and progressing through villages and events that are close by and your level and you naturally level when you go through those.
But there are areas that you need to go through for low level quests that have monsters in them that are skulls.
And 4 level difference is nothing in this game. The level difference starts being a problem if it is 8~ levels. And that is a big problem. Actually the quest log shows that quests with up to a 6-7~ level difference are green. And higher than that are red and more usual then not they are nigh impossible.
And the game is forcing linearity with level gating. That's the most primitive way to force a player through a certain path. And for your information there's other ways of guiding a player through in an open world RPG as there's a lot of games that don't resort to a level system as it is unnecessary.

There's multiple solutions to what I said like not showing the skull or monster events that you can't kill, until you reach within 5-6 levels. Going out of your way to find a corpse hag or w.e who 1 shots you, is not fun.
And not letting you get high level weapons diagrams until later in on the game, would also invalidate the need for level requirements for weapons. And if you manage to kill a monster that's a certain level, you should also be able to use a weapon that level.
And biggest one. Not making you able to recieve quests until those quests are doable (6-7 levels below the recommended level.).. Waiting for a person to die for 7 months before you get to the level is pretty immersion breaking.
And having to hack a human being or a monster for minutes before killing him is also unrealistic and immersion breaking. So if you happen in a high level area, then cutting should still deal some damage (For which the possibility of a % based damage after a certain level would be an easy fix.) and this would help us open up the world.