The 3G and 4G inter services are usually

The 3G and 4G inter services are usually provided by SIM network providers, in iPhone enabling and disabling of 3G and 4G connections can be done through changing the internal setting.

By enabling the data services you will connect and use 3G and 4G internet connection when it’s available, by disabling mobile data service you will stop using 3G and 4G but still you can connect and use internet through wireless Hotspot/Network. If 3G and 4G is disabled in your phone it will be helpful for battery life in phone.

Disabling and enabling 3G and 4G in quite different in previous 2 version of Apple’s operating system the iOS 6 and iOS 5. We have brought you the guide below of these 2 versions, follow this guide if you are not certain what version you are using in your phone. 
 

For iOS 6:

Step 1:
Locate and open the 'Settings' app.
Then from the main list of options select 'General'.

Step 2:
Go to ‘Mobile Data'.
The first two options 'Mobile Data' and 'Enable 3G/4G' control you phones connection to this service.
To use 3G/4G Internet just turn them both ON, by selecting to OFF you can stop your phone using the 3G and 4G.
 

 

For iOS 5:
Step 1:
Locate and open the 'Settings' app.
Then select 'General'.

Step 2:
Go to General options and select 'Network'.
You will see options for both 'Cellular Data' and 'Enable 3G', however it’s depending on the model of your iPhone you may just have the 'Cellular Data' option (see below note).

Read more: Enable or disable 3G And 4G on your iPhone - VUDESK http://vudesk.com/profiles/blogs/how-to-enable-or-disable-3g-and-4g-on-your-iphone#ixzz3JfUWg5vJ