What purpose does “the best part” here serve?

Normal usage is *the better part* but either way, in this context, they're saying "it didn't quite take four hours but it took way more than three hours" Your understanding should be "it took most of four hours" or "it took nearly four hours"

Used in place of "the vast majority" or "almost all."

It's a colloquial saying for "most of this time frame." So "the better part of 4 hours" might mean 3 1/4 hours, or 3 1/2 hours, or 3 3/4 hours, or some other time close to that. It's not exact because either it isn't important, or the person speaking doesn't know the exact length of time, so they just give an estimate. Edit to add examples: "It's a long drive to Grandma's house. It will probably take us the better part of 6 hours to get there." "I slow cooked that pork butt for the better part of 12 hours."

It simply means "most of" or "almost all".

To be concise, it means "the majority"

It should say, "for the better part". It means "a large part of". In this case, nearly 4 hours.

I think it should have been "better part of an hour," meaning "almost an hour" or "more than half an hour." I've never heard anyone say this; I've never seen anyone write this.

How did I guess this was a Harry Potter fanfic 😅

The best part, would infer practically as the most impactful and memorable, causing the most emotion regardless of which emotion to be above everything else.

It just means 'for most of'. It's largely only used in the context of time. I would more commonly say it's said as 'for the better part of' but I think this also largely makes sense. So in this case, you could rewrite the sentence as 'for nearly all of four hours' or 'for most of a four-hour duration'. There's probably easier ways to word that lol. Another example would be 'for the best part of two years', ie, most of two years.

*It was such a boring class! The teacher just recited the digits of pi for the better part of an hour.* *If you travel far enough North, you'll find it stays bright throughout the day for the better part of the year. Only in the last few months of winter does darkness begin to take over.* *Fortunately, the movie lasted for the better part of the flight, and by the time it finished I no longer had time to feel nervous. Before I knew it, we were back on the ground.*

Best in this case means biggest. It’s the biggest part of four hours or a large chunk of four hours. I’m guessing it’s a four hour movie or it takes place over four hours in movie time. Either way, they’re missing for most of it.

Lots of people saying they haven’t heard this phrasing to mean ‘the majority of’ but I hear it quite a lot in midlands UK

“Most of”

Looks like a bad attempt of saying "better part of", which is a cheesy way that we sometimes say "almost". 

Almost four hours

The best part of.- UK English or the better part of - US English are used to talk about a period of time that is almost whatever form of time is used i.e. hour, day, week,.month etc. It is used in a negative way, never positive,.usually to indicate time waited or time wasted. It's not commonly used by younger people, think gen x or older. It's often used in writing, again by older authors/books.

The majority. Also I've only ever heard people call it the "better part".

Definitely does NOT mean “the most fun part”.

It means “most of” but people normally say “better part”

OP's question aside, is nobody going to address "the emotional berk" a few lines down? 'Berk' is arguably short for **Berkshire Hunt**, which is rhyming slang for {something that rhymes with 'hunt'}.

“Almost” or “close to”. And it should be “the BETTER part…” The better part implies that it takes almost all of the time (“the part”) but not totally. “The best part” implies something quite different.

“Better part of 4 hours” would seem more correct, imo. But yes, as others above have said, it means “almost all of”, so they lost them for almost 4 hours.