The EX4300 version of the MutliGigabit switches (EX4300M) will be able to VC with both EX4300 and EX3400. Initial release (Q3/2018) will only VC with other EX4300 and/or EX4600 switches, but later on SW support will be added to allow VC with EX3400 as well.
Hopefully this addresses your concern.