I always go for the hag when I am 4th level, it's a challenge but can be done. Preferably sleep before entering her lair or well, you could sleep after the masks but then you have to go out of the lair to go to camp.
Magic missiles and create water are essential spells for the final battle, beyond that you can be creative.
First get past the door by walking through (there will be a conversation first with the door).
Then defeat the masks. Take down the two spell casters first ( a healer + a damage dealer), then the other two will be relatively easy. Not an easy fight though!
Get past the clouds + traps by jumping individually. You will take damage once from jumping from great height so you have to short rest or heal when all are down.
Fight the hag using magic missiles on mirror images, and to determine 'which is witch' (another trick of the hag) examine each ones, you will see there is a difference.
Try dealing as much damage to the hag whenever you get the chance. You want to take her down as fast as possible.
Good luck!
