Can you be specific about what you want to use 4G for? Protection of a connection between a central site and a remote branch?
Details of WAN bandwidth requirements are detailed in the UCM SRND, you should use those for guidance. Ultimately, you'll need to be confident about the minimum throughput level for the 4G connection and whether this will be sufficient to meet the traffic load during failover.
Ultimately the WAN technology is less important that its reliability.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...