Tracy Hickman, in the forward to CoS, says that "the vampire genre has taken a turn from its roots in recent years. The vampire we so often see today exemplifies the polar opposite of the original archetype: the lie that it's okay to enter into a romance with an abusive monster because if you love it enough, it will change...[Our hope in CoS is to] bring the message of the vampire folktale back to its original cautionary roots."
The "Lunch Break Heroes" video series goes into more detail on one way to run Strahd - as an emotionally abusive lover who systematically destroys Ireena's support network until she has no choice but to depend on him.
These are dark, adult, psychological themes here - and CoS is built to showcase them. It is still D&D, and the characters can still "win" and "beat the module" - but they should have a lot of losses along the way, and should see plenty of unpreventable death and suffering before they get there. Not every encounter is "winnable" in the sense of them feeling like heroes. In fact, most encounters should end up as compromises - victory means staying alive and slowly gathering resources and power for the ultimate confrontation with Strahd, but along the way they should experience plenty of tragedy.
So, I would suggest that the level indicators for the Chapters are not the levels at which the party can "win". Rather, they are indicators of the level at which the party can reasonably appreciate that they can't win. Or the level at which they can decide what they might be able to salvage from a bad situation. They are the minimum appropriate level for the party to experience the horror, dread and their overall helplessness to do anything about the pervading evil of the land. Or, to rally and decide that whatever they can do, whatever small victory they can achieve, is enough for now, will have to be enough. At any lower level then this, an encounter in the area will be over too quickly - they will simply lose without time to appreciate their helplessness, and the feeling will be one of frustration and disappointment rather than dread.
For example, at level 4 in Vallaki,
The characters might be able to recover the bones of St. Andral if they carefully visit the coffin-maker's shop in the day, but they have no hope of defeating six vampire spawn in combat, whether in day or night.
Likewise, they can probably
throw the fate of Vallaki into Lady Wachter's hands by killing Izek, or ensure that the Baron stays in charge by recovering the bones and doing nothing against him. But either outcome means that a tyrant remains in control and that the people of the town continue to suffer.
The module is full of situations like this - the PC's should have the possibility of marginal victories, but never complete ones.
Within this context, what does victory at Old Bonegrinder mean? A clever and careful 4th level party might be able to
rescue one or all of the children currently being held, but they shouldn't be able to defeat the hags or stop the pastry operation. The module makes this clear from the first encounter with Morgantha in Barovia - they can get her to let the child she has go just by talking to her - because she knows she will easily get more in the future, and the characters can't stop her.
I am largely in agreement with Dale M's answer said, but I do disagree about one thing. He suggested that the level might be more appropriate for combat if the party is able to separate the foes and take them out one at a time. I don't think fighting even just one of them, at least at the first level indicated, is feasible.
If the hags have any chance of losing a combat with the party, they should go ethereal at the first opportunity and then use nightmares to hound the party any time they try to sleep. Until the party is at a level where they can either do enough damage to a hag to take her down before her first action, or can dimensionally anchor her to prevent her from escaping into the ethereal, a decisive combat win is unlikely. It would mean doing over 200hp of damage in a single round. An incapacitating spell is possible, but bear in mind that the hag is immune to charm, is a fiend (so Hold Person or similar spells that target humanoids won't function), and has a +2 on Wisdom saves with advantage for magical resistance, so they would need a good deal of luck with a first-round spell.