i would have thought that 2 for $4 counts as specifying. otherwise the price would say 1 for $2.
The way it works - if it's 2 for $4, it should be the same price for 1 unit, unless they write - "must buy 2".
However, regardless if they write "must buy 2" or not, any further units purchased are at the same rate.
If they want to only discount in increments of 2, then they write - "must purchase increments of 2"