In the PC version, if you own the 2016 game on steam hitman 2 will register that and allow you to download the equivalent of the GOTY legacy pack for free. Any progress is left behind and you have to start over, but all the content is now nice and neatly in one game. If this works the same on PS4, you can buy the 2016 game and hitman 2 gold edition, and hitman 2 should see that you own both and allow you do download the legacy additions to it. If you have haven’t played any of 2016 there isn’t any progress to lose.
From IOI’s website:
"If you own a full version of HITMAN, you can download the corresponding HITMAN Legacy Pack at no additional cost.
If you do not own a version of HITMAN, the HITMAN GOTY Legacy Pack will be available as DLC for HITMAN 2 for $20.00 USD. HITMAN 2 disc owners will need to use the HITMAN 2 in-game store. Digital owners can either use the in-game store or the platform store (PSN or Xbox Store).
Players who have redeemed the HITMAN Legacy Pack can also purchase the “GOTY Legacy Upgrade” for $10.00 USD, which will upgrade their Legacy Pack to the GOTY Legacy Pack. See below for the exact contents of the Legacy Pack.
Who can redeem the HITMAN Legacy Pack for FREE?
Players who own HITMAN: The Full Experience, HITMAN: The Complete First Season (digital or disc) or all seven episodes individually (Paris, Sapienza, Marrakesh, Bonus Episode, Bangkok, Colorado and Hokkaido) from the previous HITMAN game, will be able to redeem the HITMAN Legacy Pack.
Players who own HITMAN – Game of the Year Edition (or have upgraded to the GOTY Edition) or own HITMAN: Definitive Edition, will be able to redeem the HITMAN GOTY Legacy Pack."
This might be a good way around PlayStation not having the GOTY legacy addition available.