You need to have a jail broken PS4.
It has to have old OS and firmware.
If it was updated after a specific point you can’t roll it back and it will not be able to be jail broken.
I don’t mod on PS4.
I have only ever used glitches and Save Wizard/Editor.
I mod on PC.
You can mod single player saves with Hazards Save Editor on PC.
Then you resign the save to PS4 using save wizard and upload it to a usb.
You then add the modded save to your PS4 and your single player account will be modded with whatever you did with the save editor.