You mean 4 total per flight, right (not 4 per person per year)?
Did you hear this from AS or VX? This is just a guess, but I think that their booking systems don't "share" space very well -- from looking at the seat maps, it looks like they just divide up the inventory between them (at least, to some extent). So if one of them can't upgrade you, the other one might be able to? ie, if VX told you no, try calling AS (or vice versa)
In other words, I'm guessing that this is more of a technical limitation than a policy