There is talk of needing to 'break in' new internal hubs. I know my nexus 8 speed started to shift better after a couple of hundred kms. Other than that I think the most likely fault would be somewhere in the shifter cable. This would explain the yellow marks lining up in 4th gear but slipping with others. Maybe check for compression occurring somewhere in the housing?
There is talk of needing to 'break in' new internal hubs. I know my nexus 8 speed started to shift better after a couple of hundred kms. Other than that I think the most likely fault would be somewhere in the shifter cable. This would explain the yellow marks lining up in 4th gear but slipping with others. Maybe check for compression occurring somewhere in the housing?