It's 2 separate signals effectively. Voice calls and SMS are routed over the 2G network, only data is routed over 3G. The phone SHOULD basically see them as different signals hence why it has 2 readouts - the standard signal bars for 2G and the G/3G/H/H+ etc for the data network (G is actually 2G, the other 3 are 3G).
Most phones are designed (via the software it is running) to drop back to 2G when in standby to lower power usage and extend battery life. If you keep an eye on the 3G signal indicator, it should jump back up to the H/H+ when you start to use the data connection IF the 3G signal is strong enough of course.
That is another aspect of 4G - Will the extra battery usage it uses over 3G be balanced out by the less time it needs to transfer the same amount of data e.g. 1 min of 4G takes 5 mins of 3G for same data, will the battery impact be the same? Or, for the people that need 4G (people who's job has them out in the field a lot and remote logging in etc), will they have to carry chargers and batteries with them?