Well, youre right, at 4 ohm they are noisier when compared to 16ohm, BUT also louder at the same volume when compared to 16ohm. The ohmage switch on the amp just adds resistance in the signal path.
I suspect that they put 16ohm on speakers just to make it look like the wattage it can handle is higher. So without being complicated, if you run 8ohm on your amp and you have a 16ohm speaker you should definately reconsider the 'real' speaker wattage.
I think its a linear thing, so if you have a 150 watt speaker measured at 16 ohm, it is 100w at 8ohm and 50watt at 4 ohm. So if you run your head at 8ohm, the cabinet is then for 200 watt instead of 300 watt it is said to have at 16ohm, which would be enough for a 100watt head at 8 ohm.
Personally i think one should always oversize their cab, it just the proper way IMO. Remember that putting a booster pedal in you signal path(includes a multieffect) increases the output capacity of an amp, having high output pickups will do the same.
Most manifacturers do oversize their the speakers properly, but if you mix and match stuff, SUPERSIZE that cab!
PS. You should always switch off the amp when changing output ohmage, just to be save.