It's stupid decision to lock new game plus to specific non-plus difficulty, you can't access Grounded+ without beating game on Grounded first, requiring players to beat game at least twice on same difficulty. Ideal way of doing this would be - first play through any difficulty, then you can select any difficulty for NGP.
In PS3 version there was a glitch that actually made it possible, you could do you first run on normal/hard, and instead of grinding normal+/hard+ for survivor+, you could go straight to survival+.
Someone mentioned that this bug got patched on PS3, if they kept system exactly the same for PS4 then it's really disappointing. I want to play through and do all the stuff available, like Ellie jokes/collectibles, then another play through on Grounded+ to try and beat challenging difficulty. As it stands, those who want to obtain platinum will have to play through game 3 times, 1 on easy/normal for collectibles, and two times on grounded - Grounded to unlock Grounded+ and then Grounded+ itself, and no, you can't get everything on Grounded since there is not enough shivs to open all locked doors.
Is that clear enough, or do I need to simplify it even further for you?