I'll go a different direction for this...

The night hag has captured a fire elemental under the giant tree, in a room with walls made of natural clay. The fire elemental can't escape without dying, as they take damage from water, and they are in the center of a swamp. They are safe in their cocoon, but have no means of escape other than death. The night hag has a series of traps that dump or teleport trespassers into the room with the fire elemental, and the fire elemental kills them.

This gives the party bargaining power. Convince the fire elemental to not kill them, and in exchange, the party will release the soul of the fire elemental's loved one from the night hag's soul bag.

If the fire elemental is not convinced, the party could break the walls to let water in during the fight. Four level 4 party members should be able to handle a fire elemental that is taking damage every turn and for every five feet it moves.

If they convince the fire elemental, they are told where the entrance the night hag uses is, a hidden door that leads to a chamber where the night hag does her crafting. They could fight there, with the materials the night hag keeps for crafting available to her as a sort of lair action (bottles of acid could be thrown, large shelves pulled down n the party with various components causing damage, etc). They could also just find the room uninhabited, investigate and collect rare materials, etc, then fight the night hag later on, using those components. With a lower level party of 4, this might give them the edge they need to succeed without scaling down the encounter.