It sounds like this very well my be the fault of your violin. As long as you are using enough rosin on the bow, you should be able to get a nice tone from the G string and the D string with a relaxed arm weight resting into the bow stroke. Usually it is actually easier to get a nice sound from the G string than from the higher strings, especially E.
One other thought would be to make sure your fingers are pressing the string all the way down. Again, you should be able to accomplish this by relaxing your left arm weight onto the strings through the hand. You may need to pull your left elbow in a bit to help your fingers reach the G string more easily.
Try these tips and see if they help. It really sound like you could use an instrument upgrade though, especially if you have been playing for 4 years. Nothing like the frustration of the instrument getting in the way of your progress in playing the violin!
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