Could be just one thing or a combination of reasons. A few suggestions:
Make sure you keep the strings pressed down firmly with your fingers, otherwise the string can produce a whistley sort of sound.
Remember to keep the bow position on the strings between the bridge and the fingerboard. A common habit is to let the bow wander down the strings and over the fingerboard. This reduces the effectiveness of the bow and how it makes the strings vibrate.
Use a little more bow pressure to get the strings to vibrate - the strings are quite a bit shorter when you are using fourth finger in third position, and the bow pressure, downward onto the string, will need to increase.
The strings themselves might need replacing (ask advice on this from your teacher), or it could even be the type of strings - ie. metal core, nylon core wound with aluminium etc., once again ask your teacher for some advice.
I don't know what make of violin you own, but worse case scenario is that the acoustics of the instrument itself are not up to coping with high notes. This is unlikely, but some of the mass produced factory instruments, which are designed with the absolute beginner in mind rather than the intermediate player, can be rather scratchy sounding when played in higher positions.
As I said, it could be one or more reasons, so have a chat with your teacher, and try out different things until you find the solution.
Good luck!!