I would suggest to check that all 4 ports shows up as vc-ports when executing the 'show virtual-chassis vc-port' command.
'show virtual-chassis active-topology' should also show how traffic between VC-members are forwarded.
If you are still in doubt, please provide output for the following commands:
show virtual-chassis vc-port
show virtual-chassis device-topology
show virtual-chassis active-topology