Wow, you were lucky! It took us a year to get Raini tamed. Granted she lived outside 24/7 and we didn't always see her, so we couldn't... socialize... everyday, but still. She was a wild one. She still is actually. Once, she sneaked up on one of our other cats, Snake, looking for trouble, and Snake screamed at her. It was so loud; and funny, because Snake is this teeny tiny little 6 pounder and Raini must be twice her size.I took in two ferals that I had to trap, I could not get close to them. The boy I named Rusty and the girl Kitty. Even after months of feeding them I could not touch them outside they would keep distance from me. Before I trapped them, they were getting close but not where I could touch them.
After I brought them in, it took 2-3 weeks for Rusty to warm up to me and Kitty was much longer around 5-6 weeks.
Kitty
Kitty at home.
Kitty on a little pink chair. Quite a contrast to laying on steel beams.
(Although Snake once managed to move our microwave a considerable distance. Not quite sure how she did it)
Anyways, 3-4 years of living outside is a lot. If she were a human, she'd be around 28-30 years old. So it will take longer for her to adjust than a kitten or a younger 1 year old cat. But I am positive it can be done!
Does she respond to catnip? If she does, you can tempt her out from under the couch with it. Plus, catnip makes cats feel good, and she might unloosen some of her stress. Not all cats respond to it though.