Your biggest problem is the second keyframe. You've established a constant speed between point 1 and 2, then tried to ease to to a stop at point 3. There's a time speed distance problem. It's like driving a car at 70, then trying to stop in 20 feet. You're not going to get a smooth deceleration. The way to solve this problem is to match the position of the left and right half sides of the second keyframe, pull the handle on the right side of the second keyframe as far to the right as it will go, then slide the handle on the left side of the third keyframe as far to the right as possible. The greater the time between the second and third keyframe the gentler the curve will be. As close as your keyframes are together it's going to be very hard to get more than a quick drop off similar to this one.
A better solution may be had by only using 2 keyframes. Do basically the same thing. Pull the handle on the first keyframe all the way to the right, pull the left handle on the second keyframe all the way to the right, then drag the position of the first keyframe up and down the speed graph until you get the curve you want.
One more note, if you want a smooth deceleration you don't want an s shaped curve like this. Do that and the movement will speed up just before it comes to a stop.
Hope this helps. The graph editor in AE can be a little difficult to understand especially when you're trying to do something like simulate gravity and a bouncing ball.
Think of the S curve as pressure applied to a break pedal. Increase the pressure slowly at first, then hold constant, then release a bit at the end to ease the car to a stop. This makes for a comfortable ride, but it kind of scares the pedestrian in the cross walk because if you ease off too early it looks like the car's breaks have failed and it won't stop in time. Big S curves scare your pedestrians and should be avoided if you want your animations to look smooth and natural. If you want the layer to behave like a well driven limousine, then use an S curve and to he** with the pedestrians.