I wouldn't worry to much.
I was 9 years old before somebody explained to me that 'th' is pronounced differently to 'f', and showed me how to put my tongue between my teeth. I still went on to win public speaking competitions, and present radio programmes, so a minor speech problem at 4 years old is unlikely to be of great concern.
However you might want to invent some games where you and your daughter have to make different sounds, including the 'v' one. (i.e. just have fun, without putting any pressure on her to speak properly). Then, later, (when you're confident that she actually knows how to make the 'v' sound, without being under any pressure to do so) you can gently point out how words including the letter 'V' should be pronounced.
Chris