have a Samsung Vibrant 3G and I follow the following procedure to disable data. I guess it works for 4G too. Try it.

Steps to disable data:

1. Type *#*#4636#*#* on your dial pad
2. Select "Phone Information"
3. Press "Menu" soft key
4. Select "More"
5. Select "Disable Data on Boot"/ or just disable data


Now data should be disabled (you won't see 3G icons in the notification bar) and you can use WiFi wherever it is available.
This setting resets once the phone is restarted. It doesn't save the changes permanently.
This procedure needs to be followed every time you reboot the phone. (Means - Have to disable data manually each time we reboot)
To ENABLE the data, follow the same procedure and in the 5th step, select ENABLE DATA.

======================