18 gauge. You can calculate that from the resistance numbers in this wire gauge table, by dividing the resistance number for 24 gauge by 4, and looking up the corresponding gauge.
"You don't even have to calculate since area doubles every 3 steps. 4 wires = 4 x area = 6 steps. 24-6=18"
An easier way for me to remember is that the change is the same as with dB. Double the number of (same-gage) conductors (power) and the gage number (or deciBels) changes by 3. So 2 24s = 21g., and 2 21s (AKA 2 24s) = 18g.