Why does my S4 keep going from 4g to 3g?

[Stuart Keane]

my s4 keeps going from 4g to 3g

Ive gone into settings and set it back but it does it again.

 

[Jong Lourney]

Maybe your carrier does not support 4G? If you set it in the setting "only 4G" and it swiches back, you can't do really much, phone is probably trying to save battery.