I was looking to the similar solution with my 360 camera and I found it. Even though my problem seems to be more complicated than yours. I needed some app to get data from internet while my phone wifi was being used for wireless connection with the camera. The app just always wanted to use wifi to get data and ignored all settings and even restrictions through mobiwal to use mobile connection only.
Below is where I asked about it and described a solution before I got any answers
Need wifi to be connected to a camera(no internet) and one app using 4G for internet
In your case, why don't you want to try to do it as simple as that:
1)create a hotspot on your phone where your 4G connection
2) connect your printer to the hotspot?
I did almost the same but in my case I used a second phone just as a hotspot. The reason I had to do it that way is I cannot connect to anything from the camera directly so I had to go through Wireless LAN client mode as a feature of the camera. In your case your printer should have some kind of screen that can help to connect to your hotspot.