Hi,
Good Day!!
I have few questions before i can confirm whether this can be achieved. Are you setting up proxy for internal use or for external use. Setting up for internal use is not complicated. But setting up for external users is bit complicated.
By the way we can use policy based routing and IPTables to set the desired routing to 4g USB modems.
I had used policy based routing to route internet through 4g which is a subset of your requirement.
Thanks