Sucking thumb & fingers?

i know there are different opinions and i am totally confused. my 4 months old baby has started sucking her thumb and fingers for about a week. i am trying to get her thumb out of his mouth from day 1 i found her sucking. i never gave her pacifier and she didn't need it. but by now she is almost dependent on her thumb to sleep. once she sleeps she doesn't need her thumb or fingers. sometimes in night if she wakes up she starts sucking and go back to sleep. i know she is too young and she doesn't know 'wrong' or 'right'. she is doing something which makes her comfortable. but i am worried as i don't want this habbit to continue for years..