Such a feature - of changing framerate - is called "conforming" and is often used to produce slow-motion or fast-forward like showing a plant's growth in minutes insted of days. ffmpeg do not allow overwriting/changing framerate without re-encoding. If it does so, duration will change and audio would be out of sync unless separately mended. But I'm afraid audio is not of interest in your case with framerate of .33
You want conforming because you just want to change framerate, but ffmpeg ignores
if framerate is specified in the input file. Since your input file is in .mp4 format, its own framerate take precedence of when re-encode isn't needed. For this you need a different tool: mencoder.
Assuming your input file contains no sound - likely true for any video with .33 framerate - what you need is:
$ mencoder -fps 5 -o <output> -ovc copy -nosound <input.mp4>
Notice that mencoder by default produce output in AVI format.