You can do it at the moment with nested playlists. Make a playlist called "Mixture" and mark it auto-repeat. Then inside Mixture, make a playlist called "Songs" with links to all your songs or normal playlists in (i.e. drag-and-drop in emplode, don't re-upload). Mark the Songs playlist "pick 4". Then, also inside Mixture and next to Songs, make a playlist called "Snippets" with links to all your comedy snippets in. Mark the Snippets playlist "pick 1".

Then play the Mixture playlist. The empeg will pick 4 tracks from Songs, then 1 from Snippets, then 4 from Songs, then 1 from Snippets, etc...

The only disadvantage to this is that whenever it picks one (or four), it doesn't currently pay any attention to what it picked last time, so there's a small but unfortunately non-zero chance that it'll pick the same snippet twice in a row. It'd be nice to fix this in time for 1.1, but it's not simple and probably won't make the cut.

This might be a task for "least recently played" shuffling. If the "always shuffle" would default to (or be switchable to) take the least recently played songs most likely, the chance that it takes the same snippet again would be as close to zero as possible, I guess. Depends on how strong the "prefer LRP songs" would prefer them though.
However, the overkill solution to this would be scripted shuffling, a small script language that has access to the data that is stored with the songs and playlists (when was it last played, what fid corresponds to it etc.) and outputs a simple stream of fids. This would be sooooo cooool.

cu,
sven
(MkII 12GB blue now green, #080000113)