The sum of an arithmetic sequence can be found using the formula
S = n/2(a + l) where S equals sum, n equals # terms, a equals the first term and l equals last term
we know enough of these to find the missing pieces
S = -15
n = 20
a = 4
-15 = 20/2 (4 + l) now we only have one unknown
-15 = 10 (4 + l)
-15 = 40 + 10l
-55 = 10l
l = -5.5 so the last term is -5.5
now we can use anothe formula to find the common difference
l = a + (n-1) d so the last term equals the first term (a) plus # terms -1 times (d) the common difference
-5.5 = 4 + (20-1) d
-9.5 = 19 d
d = -.5 so your common difference is -.5
