Hi - I've just had pretty much the same problem. I could not mount a qtree export using v4 but could using v3 and noticed that the volume above the qtree was also exported. I resolved this by removing all exports related to the volume and then re-exported a qtree only, which mounted using v4 no problem.
I could then also export the volume and mount using v4. I have tried to recreate the problem but I currently have not had any luck.