Over the weekend, I spent some time updating the firmware on my Verizon Jetpack™ 4G LTE Mobile Hotspot MiFi® 4510L. I don't know exactly when Verizon started using the name "Jetpack" with these mobile hotspots. I still just call my device a MiFi.
The 4510L had a problem where it wasn't switching back to 4G mode if you traveled to an area where the MiFi would switch to 3G and then returned to your home area. I don't know all the details behind the problem, but I know that I experienced it fairly consistently. I had to re-enable 4G by manually going into the setup mode and revising the preferred connection mode.
Verizon released a firmware update for the 4510L several weeks (or maybe months) ago. So, if you haven't gone through the process of performing an update, you may want to if you want to avoid the frustration of getting your MiFi stuck in 3G mode.
You can access the MiFi setup by typing 192.168.1.1 and then logging in as administrator.