Unfortunately Roblox is not officially released for PS4 but there are certain ways in which you can get Roblox on PS4. For this purpose open the internet on your PS4 and open your Roblox account rbxnow. Now go to any game and press the green Play button. You can access your account and can view your messages but cannot play the game as it does not support PS4. Therefore you will have to wait for Roblox to actually release on PS4 in order to play it. You can still however access your Roblox account on PS4 but cannot play the game.