n = %fold

EV(fold) = 3n
EV(call+lose) = (1-n)(42/46)(-2)
EV(call+win) = (1-n)(4/46)(5)

If we add up the 3 EV values and set the sum equal to 0, then solve for n, we get 31.7% (if I did my math right).

The 40% number is only right if your bet is a pure bluff (meaning that you have 0 equity on the river if called).