That's right. The iPhone will tend to select the strongest signal, regardless of which is faster. It does this to optimise battery usage, as a strong signal will mean the phone has to use less power.
Here at my flat in London I get a strong (5 bars) 2G signal, and weak (2-3 bars) 3G and 4G signals. My iPhone 6 tends to bounce back and forth between all three, which is kind of annoying.
The thing is, for data, even a weak 4G signal is still MUCH better than a 3G signal on O2. I can get 20 Mbit with a 2-bar 4G signal, but 3G will often struggle to get 1 Mbit, even with a 5-bar signal.
O2's 3G also sucks because of the long time (10 sec or more) it takes to establish a data connection, and because the data connection seems to randomly drop from time to time, requiring togging "airplane mode" in order to get it back. 4G has none of these problems!
I kind of wish I could increase the priority of 4G a bit so it would stay locked on that whenever possible instead of flipping between networks all the time. Perhaps that would mean increased power consumption, but changing networks all the time can't be great for the battery either.
