This looks a lot more like a profiling issue or coating issue on the vinyl. As in 4 pass mode it lays down twice the ink in half the time, if you haven't properly profiled for that speed, the ink will hit the vinyl and spread before getting chance to dry. this causes the dark banding seen here. I'd try creating a profile from scratch in 4pass mode, or using a media with a different coating. A possible quick fix is also to increase the temperature of your front and print heaters. More heat means a more ink receptive PVC generally.
When I see banding the first items to check are: -Feed advance -Running temps ( Put Simply Black lines = not enough / White lines = too much) -Is material compatable, are you using a canned profile or custom made?
