#1
I was going to change pots on my Les Paul and by mistake I've ordered 4 tone pots instead of 2 volume pots and 2 tone pots...

Did some research on the subject and some people say that both volume and tone pots are exactly the same, it's the way they're wired that makes their function different. Then why are they marked differently as volume and tone pots in the store?
#2
The taper is often different. Frequently a volume pot will use an audio taper, which is a logarithmic (ish) taper, because the way we hear sound means that a smooth linear change in resistance doesn't make a smooth change in volume. Tone pots are more often linear. You can definitely use them for volume pots, but usually they'll have an odd volume jump and plateau. Some people don't mind or will even prefer a linear pot for the volume, but it's rare. Audio is definitely standard for volume. 

So that's almost certainly why they're labeled differently. You're right in that they are very similar, and usually identical except the taper, but it's an important distinction. Usually the taper will be at the end of the pot code so it'll read something like 500KA for audio and 500KB for linear taper. So A is your volume and B is your tone pot. You can check when you get them to see what you have.

It's possible to use them interchangeably, or use the same type of pot for everything, but the standard is A for volume and B for tone and I think it works best that way. 
#3
When you listen to a 100dB sound and the same sound with x2 power the sound pressure is not 200dB but 103dB.

Pressure sound scale (volume) is not linear but is logarithmic.
#4
Just ordered some extra volume pots. Thanks guys, lesson learned.