it's nothing you can prevent. the phone automatically does that depending on a variety of factors. it will usually pop back to 4G almost immediately if not seconds later. it shouldn't stay on E. if it does, reboot the phone.
of course you can always adjust your network settings to maybe help it stay on 4G by selecting the WCDMA Only option. go to settings, hit the More Networks tab, then select Mobile networks and change it there. that might help but then if you get into an LTE area, you run the risk of not switching to LTE.
your choice of course