http://seoroot.com/blog/tips-and-fixes/force-samsung-galaxy-s4-to-connect-to-wcdma-only-network-mode.html

Here’s how to force your Galaxy S4 to connect to WCDMA if it keeps connecting to EDGE network mode even if it’s still able to get 3G signal from your provider’s cell site.

Go to Settings

Tap on More networks

By default, LTE/GSM/WCDMA (Auto connect) is selected. Tap on WCDMA only to connect to 3G mode only.

If you are currently using data service, you will be prompted to stop using data and continue connecting to the desired network. Tap OK button.

When you have clicked the OK button, your phone will be disconnected from the network and reconnect itself to the specified network mode.

Please note that switching network modes can drain your battery faster and can cause loss of signal especially when moving from one place to another.