Quartic polynomial

y = (x-a)(x-b)(x-c)(x-d)

Opens down

y = -(x-a)(x-b)(x-c)(x-d)

Triple Positive

0 < b = c = d -- Just pick a value > 0 amd use it three times.

Single Negative

a < 0 -- Just pick a value < 0.

a = -1 < 0

b = c = d = 1 > 0

y = -(x+1)(x-1)(x-1)(x-1)