This is ridicules , i went to T mobile and asked them why Note 4 doesn't support Group SMS feature but Note 3 does? they said its about framework and as soon as Note 3 users upgrade frameworks they will no longer have group texting feature.. I asked him why? he said because Google wants to all of his users use hangout... i am about to return my note 4 and go back to iPhone. this is not acceptable
The information you were given is about as accurate as a Politician's answer.
The Note 4 absolutely supports group MMS. The stock app does it reasonably well, Textra, Chomp and other 3rd party apps support it even more. It doesn't always cooperate with iMessage (iPhones) but that is not a flaw of the Note 4 or Android, that is iOS not willing to be compatible with other platforms.
If your phone isn't supporting Group MMS even after changing the settings to allow it, I would have someone at T-Mo Tech support look at the back side of your account provisioning, do a factory reset of your phone, or exchange it out.
It is absolutely, completely false that Samsung has made these phones not work with group MMS so that Google can get everyone to over onto hangouts. Samsung has no dog in that fight, and it would make their devices look bad, not Google. No way they're going to touch that with a 10 foot pole.