Hello and welcome to PFK! Congratulations on your new baby girl Lea!
Keeping her in your room is fine. It's the one room that is the important thing, instead of many rooms. Don't try to force interaction with her. Hissing and growling is just her way of communicating to you that she is scared and uncertain.
How often are you in your room? Do you have to be away for long periods?
If she comes out when you are on your bed, that is a good thing. Talk to her in a soft voice so she can get used to you. Use her name a lot and tell her how much you love her, how happy you are that she is there, and how much better she will feel in a while.
Very important for you to know that four days is a
very very VERY short time, for any cat. Kittens sometimes do adjust quickly, but not all kittens, as you are discovering. Some need weeks to adjust. You are going to need to be very patient, but I assure you the rewards in the end, the feeling of winning a tiny innocent animal's trust, well it's indescribable.
You and Lea will have a really incredible bond. But you must be very patient and go at her pace.
You'll see
I'm just off to bed but wanted to welcome you and Lea and to urge you to stick around and keep us posted on Lea. I know the others will be along sooner or later to chime in.