#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <cstdio>
#include <ctime>
#include <assert.h>
#include <chrono>
#include <random>
#include <numeric>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

constexpr ll mod=1e9+7;

ll mod_pow(ll a,ll b){
	a%=mod;
	if(b==0)return 1;
	if(b==1)return a;
	ll res=mod_pow(a,b/2)%mod;
	res*=res; res%=mod;
	if(b%2)res*=a;
	return res%mod;
}

struct perm{
private:
	int sz;
	vector<ll> p,invp;
public:
	perm(int n){
		sz=n+1;
		p.resize(sz),invp.resize(sz);
		p[0]=1;
		for(int i=1;i<=sz-1;i++){
			p[i]=p[i-1]*i%mod;
		}
		invp[sz-1]=mod_pow(p[sz-1],mod-2);
		for(int i=sz-2;i>=0;i--){
			invp[i]=invp[i+1]*(i+1)%mod;
		}
	}
	ll comb(ll x,ll y){
		if(x<y||y<0)return 0;
		return (p[x]*invp[x-y]%mod)*invp[y]%mod;
	}
};
perm p(1<<20);

int main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	ll a,b; cin >> a >> b;
	ll n,k; cin >> n >> k;
	a%=mod;
	b%=mod;
	if(k==1){
		printf("%lld\n",a);
	}
	else if(k==n-1){
		printf("%lld\n",b);
	}
	else{
		ll res=p.comb(n-1,k-1)*a%mod+p.comb(n-1,k-2)*b%mod;
		printf("%lld\n",res%mod);
	}
	ll res=0;
	(res+=a*a%mod)%=mod;
	(res+=b*b%mod)%=mod;
	for(int i=2;i<=n;i++){
		ll u=p.comb(n-1,i-1)*a%mod+p.comb(n-1,i-2)*b;
		u%=mod;
		u=u*u%mod;
		(res+=u)%=mod;
	}
	printf("%lld\n",res);
}