import sys
input=lambda: sys.stdin.readline().rstrip()
mod=10**9+7
a,b=map(int,input().split())
n,k=map(int,input().split())
a%=mod
b%=mod
n_max=4*(10**5+1)
F,FI=[0]*(n_max+1),[0]*(n_max+1)
F[0],FI[0]=1,1
for i in range(n_max):
  F[i+1]=(F[i]*(i+1))%mod
FI[n_max-1]=pow(F[n_max-1],mod-2,mod)
for i in reversed(range(n_max-1)):
  FI[i]=(FI[i+1]*(i+1))%mod
def comb(x,y):
  return (F[x]*FI[x-y]*FI[y])%mod

if k==1:
	print(a%mod)
else:
	print((a*comb(n-1,k-1)+b*comb(n-1,k-2))%mod)

ans=((a**2+b**2)*comb(2*(n-1),n-1))%mod
for i in range(n-1):
	ans=(ans+2*a*b*comb(n-1,i)*comb(n-1,i+1))%mod
print(ans)