H,W,N,K = map(long,raw_input().split(' '))
S = H*W
a = [i for i in range(S)]
n = 1
for i in range(len(a)):
	a[i] = n % (N+1)
	n += 1
	if n > N:
		n = 1
if a[S-1] == K:
	print "YES"
else:
	print "NO"