Exchanging XiD boxes (HD) for XG1v4 / Xi6 boxes (4K)?

My Equipment:

I have three XiD STBs (HD) and one XG1v4 STB (4K) in my home, which all receive signal via coax (upstream) and are connected to 4K TVs via HDMI (downstream).  

I recently purchased my own modem (MB8611; approved by Xfinity) and router (Nest Wifi). I would prefer to keep this setup and avoid going back to renting a Xfinity Gateway (e.g., TG3482G). 

Goal:
I want to upgrade/exchange the XiD STBs for 4K compatible ones (either XG1v4 or Xi6 boxes).

Questions:

Can I exchange the XiD STBs for additional XG1v4 boxes? If so, what is the best way to obtain additional XG1v4 boxes. (Note: I prefer this method because the XG1v4 boxes utilize coax and, thus, would not require changes to my modem/router). 

If not, can I exchange my XiD boxes for Xi6s without requiring changes to my current modem/router (i.e., needing to rent a Xfinity Gateway)?