Can an EX3400 and an EX4300 be used in a VC configuration?
EX3400 devices cannot be connected with any other platforms to form a VC:
QFX5200, QFX5120, EX4650, EX4400, EX3400, EX3300, EX2300, and EX2200 switches cannot be mixed with any other models of switches in a Virtual Chassis.
Reference link: https://www.juniper.net/documentation/us/en/software/junos/virtual-chassis-qfx/topics/concept/virtual-chassis-ex-qfx-series-mixed-understanding.html
For Juniper Networks EX3400 series