Don't give up. I am 4"11 and I Mt Biking.
It has been somewhat hard to find a bike that really fits ,make sure you find a shop that has a good fitter and is willing to help. You might have to go back in a few times till they know you really (are looking at buying a bike.)
I have had two Specialized that fit pretty good both (Specialized Myka's in a womens small ) first in HT ,then up graded to the FS. I liked both bikes and as I was just getting Mt Biking they worked real good. (I did notice the differance between HT and FS)
I road a Giant in the womens small but it just didn't feel right ,They couldn not get it dialed in for me.
I now have a Womens (Trek Lush SL) FS size 14 1/2 and I love love love this bike .
Do not give up and keep going back till you find a person that is willing to help you and make you feel like they want your business. Ride as many bikes in your price range as you can before buying one and do not buy one if you cant try it out first.
Being as short as I am trying out a bike first is key because on some they just do not have enough room to tweek things.
Best of luck to you.