First of all, as Jiisuki mentioned, you should try for six frames.
The sprite doesn't come off clearly as a banana. It could easily be a floppy piece of rigatoni. Nor, and this is more personal, do I think a banana would stand at that angle if it had legs.
With that in mind, here are two attempts on my part:
I kept the same number of colours as yours, and they're only slightly larger than yours.
I didn't add arms because I was having trouble wrapping my mind around the motion. Because of the lack of flexibility in a banana the arms would have to move at the same time as the respective legs, as in a cross country skiing motion, as opposed to the more natural feeling arms moving opposite the legs.
Hope this helps.
