How to add 4G to device?

4G is a network protocol, not a device. And actually 4G is also called LTE; they are the same thing, unless your carrier is AT&T, who call 3G HSPA (fast 3G) “4G” for marketing reasons. So there is actually no such thing as a 4G network. And all US carriers (and many non US carriers) are shutting down their 3G (and thus “4G”) networks. AT&T and Sprint already have; T-Mobile and Verizon will in the near future.