I hesitate to respond here, as I was not 100% sure of the answer, still am not, BUT I believe the MACSEC functionality will be the same between EX3400 and EX4300. Supported on all interfaces be they 1G or 10GE, but not on 40GE interfaces, 128 bit AES encryption. At the same time, I believe there is not yet SW support (HW is capable) for this functionality on EX3400. I also heard that maybe EX3400 will be able to support 256 bit encryption in the future, not so sure about EX4300.
I am still trying to confirm all this 100%. If/when I do I'll update this thread.