I was out of town this weekend. In the past if I was experiencing poor reception I dialed *228 option 2 to update the phone's local PRL. This would then update the list of towers the phone was allowed to roam on. Well on my new GS4, after *228 option 2 I was stuck on 3g. Customer service confirmed that *228 option 2 on a 4g phone can mess with the sim card. He said sometimes it can get completely wiped or in my case prevent it from connecting to the LTE network. They offered to ship me a new one, or I could go to a store for a replacement. I'll be going to the store tomorrow.
Only after being forced to use 3g for a day now, I really miss LTE. It is amazing how much slower everything is on 3g.
For future reference, updating the PRL on a 4G phone is handled by the SIM card and is usually done automatically. If you experience poor reception you can try a manual PRL update by turning off the phone, removing the SIM card, waiting a few minutes, replacing the SIM card, turning the phone back on, then allowing the phone time to reconnect to the network.
And be sure to go to a Corporate Verizon store for your replacement SIM card; they are free. I believe they usually charge for them at retailers.