You can change the speed of your video using setpts video filter.

To speed up your video, you can type:

ffmpeg -i input.avi -vf "setpts=0.5*PTS" output.avi

Note that this method will drop frames to achieve your desired speed. You can avoid dropping frames by specifying a higher "output frame rate" than the input, for example, to go from 4 to 16 fps:

ffmpeg -i Input.mp4 -r 16 -vf setpts=0.125*PTS -an Output.mp4

Or possibly by specifying the vcodec as "copy".

To slow down your video, you can type:

ffmpeg -i input.avi -vf "setpts=2.0*PTS" output.avi