We have always followed the JTAC recommended level for Junos found here:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB21476&smlogin=true
That is what led us to use the 15.x code on the EX4500s and EX4200s. I do see that the recommended code for the EX4600 is 14.x. We will be ordering the EX4600s later this week and when we get them on our bench we will check back with JTAC for the recommended level of Junos.
Attached is my EX4500 config (with passwords blanked out). I did run this through the ELS translator and it did not find any code it could not translate.