Is the port on the EX4300 configured to be a virtual chassis uplink?
show virtual-chassis vc-port
If so, delete it.
request virtual-chassis vc-port delete pic-slot 1 port x
Is the port on the EX4300 configured to be a virtual chassis uplink?
show virtual-chassis vc-port
If so, delete it.
request virtual-chassis vc-port delete pic-slot 1 port x