An older EX4200 should operate in the same manner as a newer one, so I wouldn't buy newer EX4200 (maybe EX4300 if you feel like you want something "newer"). You may want to check the bootloader version to be sure it is up-to-date, but as long as that is upgraded, your "old" EX4200 should run any supported code. The only other thing that you might consider is swapping out the uplink module for one that uses SFP / SFP+ as XFP modules tend to be pretty expensive, especially when compared to using direct-attach cables. I would of course suggest getting a second power-supply for each unit if this is a critical application.
-Ron