They did at some point in the episode (I don't remember if it was near the end or not), Hershel was talking to Rick, and the camera went down to Hershel's leg, showing the stump and the prosthetic leg next to it.
Yes. They found an artificial leg in the timeskip between Season 3 and 4. It's shown twice in the episode. Once when they have the Rick and Hershel checking out the garden's crops and then again when they are sitting around the table and Hershel removes the fake leg to massage his stump.