I have a 2x12 speaker cabinet, with 2 x 8ohm speakers in parallel for a total 4 ohm load. My amp head can deal with 4 ohm load, as well as 8 ohm load.

So, what I would like to do wire the speaker cab in such a way that I can toggle a switch for 3 possible options:
1) Speaker1 only (this is a 8 ohm load)
2) Speaker2 only (8 ohm load as well)
3) Speaker1 + Speaker2 simultaneously in parallel (this is a 4 ohm load).

The speakers are voiced differently, so I like the idea of mixing/matching like this on the fly at the flick of a switch. (no silly AC/DC references here)

Can anyone experienced with this provide a wiring diagram to achieve this, and what the correct type of switch would be?
yeah, it's possible. you'll need a 2P3T rotary switch.

I don't feel like making a diagram, but I'll gladly proofread one that you make.

HINT: Telecaster switch wiring
define 'on the fly' for me please. because if you are saying you want to be playing and switch speakers mid chord or whatever, i wouldnt think that would be a good idea. youve got that moment where the switch is in between loads and your amp sees no load. or youve got arcing when switching. or maybe im just used to things with a heck of a lot more power/current going through them and this wouldnt really be an issue.

if you were thinking more along the lines switching between songs when nothing is going through the speakers, that should be fairly simple. a 3 way pickup style switch could do the job most likely, and 2P3T switch like Jim mentioned would work quite nicely