I would like to buy an RPi4. But I wonder if LibreElec would benefit from a Pi with more than 1GB RAM?
As with all computing stuff, buy the most you can afford at the time.
1G Fine for basic use - All previous RPi had 1G or less and they all worked fine.
2G Ideal if you can afford another $10
4G Probably overkill - but it's only another $10 and it allows you to run more addons and other stuff - TVH etc.
Remember that in 2020/2021/2022 (Whatever) there maybe a RPi5 so your choice is only going to be short term.
I'd go for the 2G as is satisfies most solutions.
For example if you use Tvheadend you could use 3gb RAM as timeshift if you choose the 4gb model
But I would always choose at least 2gb if use LE, just to have a bit room for future things.
4gb are great but just for LE not needed at all