Thinking of balance is a good way to judge by eye whether your figure looks right in its current frame of motion; if it looks like it couldn't possibly hold that position for a second at the momentum depicted in the scene, then there's probably something a bit wrong with it.
In the third frame the balance shifts a bit--the forward leg straightening a bit more and thus capable of supporting more weight, while the backward leg begins to lift off the ground and come forward. Here you can use the second and fourth frames to help you estimate that position, by looking at halfway points between the knees, the joining of the upper legs, the heels of the feet.
One thing you'll want to remember is that the knees, etc. won't be at the same elevation for each frame, because the body is dipping up and down and the legs are bending.
