i have the same problem .ive heard its now discontinued due to problems.seems its best to upgrade to 8 speed set up.
I have found the actual hub very reliable, all the problems have been with the shifter were the indexing is done. Have you checked the shifter is adjusted properly? On top of the RHS of the hub by the bit that the gear cable is attached to is a little window. Check that the two red lines line up.
shimano nexus 4 speed gears hub?
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