sorry , that accidently accepted the solution.
once you connected a device to the interface of 4300, can you login and check whether that interface is added to a vlan or not , also see >show interface ge- 1/0/x , you will see all the details there . please post it if you still dont see it , secondly the 4600 is ELS and it allows irb instaed of VLan so creating vlan should be as below , also see the spanning tree
set vlans VLAN56 vlan-id 56
set vlans VLAN56 l3-interface irb.56
set interfaces irb unit 56 family inet address x.x.x.x/24
set interfaces ge-0/0/x unit 0 family ethernet-switching vlan members VLAN56