i honestly dont know 100% if you can toggle between them, but i'm pretty sure you can't. are you sure you're on 4g to begin with? you can try to take out your battery and leave it out for a minute and then restart the phone. i had a problem where it wouldnt connect and that fixed it for me. if that doesnt work for you, try flashing a new modem. if you're at home, just try connecting to wifi; it may just be your area.
If I helped you, the thanks button is very much appreciated.