I have a SAP V4 module but with COMAND Online.
And I have seen both 2G and 3G connections.
The SAP V4 module even changed from 2G to 3G and vice-versa if there is 3G available or not.
I started the car in a closed area (parking space two floors below ground level) and the display showed 2G. This is to be expected because below the surface the phone would have a 2G only connection to the network anyway.
After I drove out of the parking area the display changed to 3G. Because now without walls the SIM card can get a UMTS (3G) connection.
Thies proved to me that the SAP V4 module tries to get the fastest connection (3G) but if I am in a location where only 2G is available than it switches back to 2G. It behaves just like a phone.
If you suspect a firwmare isse than update the firmware of the module. There is an update available.
I have updated mine. I didn't have any issues but after I found out there is such an update I installed it anyway.
Look at http://www.mercedes-benz-mobile.com/#/gb/en/passenger
Select there your car and you should come to a site from where you can download the latest firmware for SAP V4 Module and some instructions how to update.