Children usually suck thumb or fingers during their infancy..
And you have mentioned that she is doing it only for the past 4 months..
It looks like that something is bothering her and she is unable to discuss and share with your family members..
Next Steps
Please seek a professional support so that she might be able to open up and get rid of the issue by their guidance.
Health Tips
For a detailed discussion and understanding you can reach me by using the link given below :-
https://prac.to/hema-sampath-psychologist-dir