The total number of parts to the paste is 4+3+2+1 = 10. Of those, 2 parts are resin. If the "parts" are equal weight, then 2/10 of the weight is resin.
One billboard requires 30 pounds of paste, so 4 billboards will require 4·30 = 120 pounds. If 2/10 of the paste is resin, then (2/10)·(120 pounds) of resin will be required, or 24 pounds of resin.