It's not ideal from a number of factors. I shared a room growing up....with a similar age sibling...and then had my own room from around the age of 12.
I don't think sharing rooms is a problem per se but 4 in a room is going to be crowded - if the room was enormous, presumably it would have already been split.
I can't fathom how bedtimes would work with a 1 year old, 14 yo and 2 in between for a start
Secondly there is no privacy for a boy going through puberty
As well as not having quiet space, space for toys (younger kids) or to study (older kids)...or just be alone or quiet
I think though posing the question as AIBU, lots of people are desperate to say yes, YABU regardless of the details!
Having said all that, while I do think it's unfair that 4 siblings are sharing and potentially irresponsible of their parents to set up that situation, presumably they just have to make the best of it, and find ways to make it work. If it's not your own family that's not a lot you can do about it!