I'm just short of 4'11" and riding a hybrid too. In my case I am riding an extra small Cannondale Quick with 26 inch tires. It is smaller than your GT ZuM, especially the reach, which is also a problem for me. Eventually I'll probably get a road bike. The 44cm Jamis Xenith Endura seems to fit as does the 44cm Specialized Ruby. Both are kind of pricey. If they don't work there always is the Terry line of bicycles. Trek makes one very small road bike at 43 cm in it's Lexa line, the SLX. It is about $1400. It has smaller, 650 wheels.
I would hunt up 42-44cm bikes to try.
What I have found difficult is finding a dealer with small bikes to try. Some want you to pay without trying to special order. Big cities seem to have more available.