Hi Daniel.
Yes, a persistent world is definitely possible with Photon.
However you will also definitely need to dig a bit into C++ and C# coding to achieve what you want (or let Exit Games do all the Photon related coding work for you if you are willing to use our consulting services).
See Turn-based & Async. Games | Photon Engine, Room Persistence Guide | Photon Engine and Cached Events | Photon Engine for more information.
I have linked the according chapters of the Photon Realtime doc, but those features are also available with Photon Server, the according pages have just not been added to the Photon Server docs, yet.
For interaction between Photons server side and UE please see Photon Plugins Manual | Photon Engine, Photon Plugins FAQs | Photon Engine and http://doc.photonengine.com/en-us/onpremise/current/plugins/plugins-upload-guide.
If you want to run UE 4 instances on the server side, then you need to either choose Photon Server (you are responsible for hosting the servers) or subscribe to a Photon Enterprise cloud (Exit Games does the hosting for your on dedicated machines) as Photon Public Cloud (Exit Games does the hosting for your on machines that you share with other customers) does not support custom server side code.
Note that when doing networking through Photon (which is likely the simpler approach compared to using UE networking, but store and load the state of the game through Photon) you can not use UEs replication system, but would send ingame updates through Photons opRaiseEvent() instead.
There is also Photon Swarm, a solution which uses Photon for matchmaking and LoadBalancing, while using UEs builtin networking for in game communication. This would be an excellent starting point for you as it already spawns headless UE instances on the server side. You would just need to adjust the code to not let those instances communicate directly with the clients, but with Photons server code, so that the clients would communicate solely through Photon and you could therefor use Photons persistence features.
Photon Swarm is currently available on request only. Drop us an email (Contact Adresses and List of Events | Photon Engine) to get it.
It’s probably a good idea to get in touch via email anyway if you have further questions, as our server engineers and architects usually don’t look into the Unreal forums.