4 months old is still very young, and if you've only had her for 3 weeks, she's still getting adjusted to her new home. It may be too large of an area for her to always remember where to find her litterbox.
Are you crate training or does she get run of the house? I wouldn't give a 4 month old run of the house if that's the case.
When you can't always keep an eye on her, you might want to keep her in a crate or a smaller room with her litterbox. A small room or bathroom with tile floors will make clean ups easier if she has an accident.
Another thing is, you want to praise and reward her with a small treat when she uses the litterbox. Tasty treats will always work better than punishment for any kind of training. |