I'm not too familiar with 4G tech - but why "move back" to 3G? If the LTE network falls over, your phone switches to 3G and keeps right on humming along more sl

I'm not too familiar with 4G tech - but why "move back" to 3G? If the LTE network falls over, your phone switches to 3G and keeps right on humming along more slowly, doesn't it? Or is my concept of 4G modems out of date?