A long and detailed discussion of why the night hags are a "more than deadly" encounter follows: just warning you in case you don't care!
So, it's definitely not exactly like an adult blue dragon: that's a situation where a level 4 party has no chance and will all die pretty much 100% of the time, outside of constant natural 20s on every roll. There is at least one viable path to victory with the hags that I can think of, although it still requires some luck and good planning (and possibly ignoring one of the monster's key abilities).
With a complete tactical knowledge of the situation and the creatures being faced and plenty of time to think about it (none of which the characters would likely have), the best chance I can come up with against the hags is a surprise Silence spell - it's on the cleric list (you DO have a cleric, right?) so it can be prepared, and it curiously has no save (which is great because they get advantage on saves). It's also ideal because it shuts down the hag's best options (casting) while also plausibly allowing the characters to fight her without drawing the attention of her daughters, who are about 10 feet away (maybe closer to 20 if you can get her on the ground level - also nice because there are no windows). Also, it's one of the ways the tiny combat area works in your favor - it can easily lock down a whole floor.
If they can isolate the leader (easy enough) and silence that floor (again, auto-works) AND then block her from going up the stairs or otherwise leaving (the hard part), she's actually a reasonable threat: reduced to simple melee attacks, it would take her an average of 8 rounds to kill a level 4 party meatshield. She still has awesome damage resistance (hopefully they have more than 1 silvered weapon by now), great AC, and a ton of HP to chew through, AND you're weakening your own casters, but at least you have a chance - until she shoves your party out of the way (18 Str) and goes and gets her daughters. If you have ways to knock her prone or otherwise lock down her movement, that's ideal (grappling her will still be difficult - 18 Str). Assuming the party was fully rested and novas (don't forget the cleric is concentrating on the silence, though) it should be possible for them to take her out before she downs one or more of them (again, assuming she's unsuccessful in escaping and doesn't specifically target squishies in that tiny, tiny combat area). I could see an optimized party of 4 (with multiple silver weapons and Ireena in tow) doing it in about 3 or 4 rounds on average.
But all this does is remove one hag from the equation - although that's enough to break the coven, making the remaining two night hags "only" more than 2.5x a "deadly" encounter (instead of 4x - or 8.5x for all three!). A similar tactic might work on the daughters, although you'll likely want to come back tomorrow - but fortunately they seem to be described as a bit dumber than the leader and it's probably ok for the DM to be less tactical when playing them (like having them investigate what's up one at a time, for example). And without their coven spells they mostly just hit you (sleep is their worst effect and that's really just an action trade, unless they can catch several characters at once).
This is still a best-case, though. Otherwise the hags have 9 chances to lock down up to 3 targets (polymorph, hold person, bestow curse - all DC 15) and while it might take one hag an average of 8 turns to kill one level 4 meatshield, it won't take 3 hags more than 3. (Fortunately the area is so small I'm assuming they'd never use Lightning Bolt, which could easily kill one or two characters.) One thing you can do to tone down this type of encounter - if it comes to this - is rule that when a coven is broken, not only can they not cast coven spells, but all current coven spells end. It's a reasonable interpretation and would likely help a lot.
This is all ALSO assuming the night hags haven't bothered to keep up with their heartstones, which makes them effectively immortal since they can just leave at any time on their turn. If you include that in the mix, Silence is really your only option - assuming the DM rules that it also ruins the Etherealness action, which is reasonable to interpret either way. If they have that ability you should never be able to kill them unless you can somehow find the object on their person and remove it (they don't sleep) or do massive damage in a single round (might work on one of them).
Interestingly, switching the group to green hags doesn't actually change much: it just removes 30 HP from each hag (maybe one round's worth of damage) and their insane damage resistance (which mostly matters if you don't have silver weapons). It also lowers their coven spell DCs by 2. Also, instead of ethereal, they go super-stealth invisible at will (easier to contain). They're actually even a bit more thematically appropriate (since they like "bringing people low and seeing hope turn into despair" instead of just killing them really slowly in their sleep), although their Illusory Appearance is less powerful than the night hags' Change Shape and allows a save to disbelieve. Still, they're much more approachable for a 4th level group. And for lowering the risk you also lower the reward: the party doesn't get three rocks that can cure any disease forever, either.
