First of all I have to say that what you're talking about is not at all related to the fast dormancy. What you're talking about is your phone's reaction to the low signal, it has to be like that because maybe hspa and h+ needs a strong signal, but your and my phone are very sensitive to the low signal and they switch to normal 3G(UMTS) even when it has 4 bars(1 bar less than full).
One way to limit your phone to stick to hspa and never fall to UMTS 3g on my phone sgs3 is to get into the hidden menu by this code *#0011# and then go to network settings and limit my network frequency to 900MHz.
Az I've had experiensed it, it only shows H as HSPA ! Even when you have 1 bar.
But unfortunately this menu is limited on android 4.3, so you can't go back to control your phone's network settings.
Hope you find a way, because it really makes me crazy when it switches to 3G everytime you get 4 bars or less