Thanks for the help, i kinda know why the interfaces don't work.
It's because the VC is not supported using just one EX4600 and all the other members EX4300
In case of failure of the master (ex4600) only the same model can take the role of master.
So that means i need to get another ex4600 
Anyways i just created a vc between the two 4300 and put the 4600 connected to an ex port.
if you want to know more go to http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/reference/specifications/cable-ex4600-virtual-chassis.html
note: i did tried to configure the chassis with version 14.1X53-D16 it only supports one switch of each model. when i added the other 4300 the whole chassis becomes a non mixed one, so only the two 4300 are recognized in it.
Giulianna