If you can paint the room some fun but calming colors or decorate with certain colors. Light blue is supposed to be calming.

Use calming colours, pastel like a pale blue, turquoise, lilac, lemon or pink. Don't make it too fussy and don't have too many toys/games in there but have some familiar things she's attached to. Let her choose some of the things too....from a limited choice of yours, lol
Hope it works. I think you'll need some patience judging by her age.

I'm thinking a smooth white noise machine (which they also have free apps you can download, but something nice like running water).

Also maybe a night light, or you can do nightlight stars


Or a light projector of some sort.

Lights so if she gets scared she can see. This time of year I have seen purple light strings, like Christmas lights.

A mural with lights

I like what people are saying about lights. Perhaps those LEDs that change color. But whatever ideas you go with, make sure she has input even if it's you telliong her "you're deciding if it's A or B".

at that age, I'd have been thrilled if someone would have just asked me what I wanted and then tried to help me bring it to fruition. that said it might be a very difficult thing to make happen and might not do well for making the place easy to resell or might not be something your landord is ok with, if you don't own the property.

Glow-in-the-dark stuff on the walls. Whatever her favorite shapes would be: animals, fish, whatever.

Glow-in-the-dark ceiling stars are the shit.

Colour changing Led lights. A tv. Nice bedding.

Let her decorate her room

Ask her.

I love the idea of a forest theme. Lush layers of multiple shades of green full branches trees & maybe a horizon or mountains in the distance.some animals or critters too