Converting a 7/8spd cassette to 4/5spd Lets get some things ticked off:


Friction shifting, check
Narrow stance cranks, check
2 or 3 spacers on the right hand side, check
Single chainring, check


(That's the context.)


Are there cassettes which can be disassembled completely and reordered?
If so, what type/what manufacturer/what time period/what product line?
If you take out cogs 6, 5, 3 from a 7spd cassette, is there a derailer capable of handling that range?


... Anything I haven't thought of?

edit: I just thought of something else... The spacers may not be such an easy part to get hold of. The spacing is all completely different. Would 4 7spd cogs + 3 of what might be called '7 to 8spd' spacers all fit together nicely on a 7spd freehub? This is what I'm not sure of.
Because you can does not mean you should, however as an experiment, if it does not work you can always put the cassette back together as it was in the factory. 

Most Shimano cassettes that don't use spiders can be disassembled (usually cheaper ones). They are held together by either rivets or small screws with hex head. You can drill out the riveted one easily enough - these are no needed once the cassette is installed. 

With the right spacers, it would be possible to custom make a cassette with 3 or 4 cogs and any sizes you have availble (Do make sure the cogs a placed in ascending order, although ). The shifting won't work smoothly, as the ramps and stuff on the cogs will be in the wrong places.  

Refer Sheldon Brown - section "Building Custom Cassettes"