Same number of frames as original.
It isn't enough to simply add frames to make an animation look 'smoother' -- If your animation is broken at 4 frames, 100 frames of transition won't help it look any better -- it'll just be slower, and speeding its display rate will lead to the exact same broken animation you had at 4 frames.
The problem with 'jerkiness' is actually in the spacing 'between' frames.
Notice that I said "
between frames" -- this is important because most of your animation isn't even drawn -- it is simply implied.
When treating your low-frame animation as separate and disjointed images, you will almost certainly experience "jerkiness" in your animation, even when using onion skinning to help you with spacing because you're probably not taking into account mass/weight, silhouette, 3d-arcs, or, most importantly, basic physics.
These things are scientific variables that can be calculated and predicted, and our eyes/brain can do that calculation without us even being aware of it consciously. Even people who don't know anything about physics or animation can still see there's something wrong.
In a way, you have to backtrack that mental calculation (between frames) when animating and ensure stuff lines up with the physics you're trying to convey. This ensures the jerkiness will vanish since everything is following a natural rhythm and locomotion which makes sense to the viewer's brain and eyes, leading to a smoother-looking animation because the brain is not simply discarding the useless and incomprehensible portions of the animation frames which lead to the 'jerkyness' you're experiencing when viewing the animation.
Also keep in mind the silhouette of keyframes.
Each frame that has some sense of visual 'impact' should be considered a keyframe, and the 'key' to the keyframe is a strong iconic shape that is clearly recognizable through shape alone.
As you're intending in your animation's usage in-game, each frame has a purpose, and should have some sense of dynamic impact in the silhouettes since they will all be held at some point for longer than a frame (or so I would assume.)
A good point of reference for strong silhouettes and a low animation frame-count is Anime. Your stuff is more Warner Bros. / Looney-Tunes style, but the principles of physics and strong dynamic keyframe poses still apply.
Frame 3 was your strongest offender here since his hands crossed his face in this pose. If the frame were completely black, you'd have a hard time telling what was going on vs. the pose in my new edit.
In this version of my edit, I tweaked the Frame 4 silhouette a little and added an outline to the hand to make it appear more visually clear that it is on a completely separate 3d plane than the face (the style of spriting you're using isn't very common due to problems like these that require outlines to be used in some circumstances such as the one here -- and although usually a number of colors with high contrasts can solve this issue as well, in cases where a skin tone on a hand must go over a face with the same tones, or in other cases where there's no brighter colors on the palette, an outline of some kind is essential for clarity.)
Anyway, that should give you a good start on your animation stuff.
I modified the belly a little for anticipation/recovery/3dform to emphasize the blobby-ness of your guy, and kept the moobs female-looking. xD
