The Samsung Galaxy Note 4 is still considered to be up-to-date as it is currently running on its August security patch (in Europe). However, this may all change once the Galaxy Note 7 picks up again in 2017, leaving the Note 4 to slowly, but surely, become obsolete, just like the Note 3.
But the Note 3 has a solid excuse; it runs on the Snapdragon 800 which isn’t compatible with the Android 7.0 Nougat update. This means that it can’t be updated to the latest software, unlike the Note 4.
The Note 4 does not face such problem as it runs on a Snapdragon 805 chipset which is compatible with Nougat. Samsung seems to be mostly at fault for this.
The Note 4 doesn’t seem to have much choice left without software and security updates. It is reaching the end of its line and will have to soon bid farewell to Android.
At this point users don’t really have any nicer alternatives. The Note 5 that directly precedes it lacks in an expandable storage, and the Note 7 is dead. So, there’s really no other options left besides wait on its impending death.