I recently purchased a 4600 and within an hour of stepping aboard as her new owner experienced the same problem you described while fueling her.
I was told that while heeling fuel may creep up the vent line and create a vacuum block. My remedy (don't laugh because it worked) was to put my mouth up to the vent and blow air back down the line.
Long term, I was advised to trace the line to be sure that all areas slant up so as not to create a pocket for the fuel to collect.
Hope this helps.
Kevin