If I read this documentation correctly, NSSU is not supported on a EX4600/EX4300 mixed VC: https://www.juniper.net/documentation/en_US/junos/topics/concept/nssu-ex-series.html
The page is updated back i January 2018 but I think you need to raise a JTAC case to validate if the documentation hasn't been updated correctly. I see several occurences where EX4600 is mentioned in the section headline but not in the figures or bullet lists.... so I could be supported but not listed.