// Slower C++ solution:
#include <iostream>

#include <cstdio>

#include <cstring>

#define int long long

#define fre(x) freopen(x".in","r",stdin),freopen(x".out","w",stdout)

using namespace std;



inline int read(){

	int sum=0,f=1;char ch=getchar();

	while(ch>'9'||ch<'0'){if(ch=='-')f=-1;ch=getchar();}

	while(ch>='0'&&ch<='9'){sum=sum*10+ch-'0';ch=getchar();}

	return f*sum;

}



const int N=200055;

const int Mod=1000000007;

int n,m,A,B,ans,jc[N]={1};



inline int fpow(int a,int b){

	int ret=1;

	for(;b;b>>=1,a=a*a%Mod) if(b&1) ret=ret*a%Mod;

	return ret;

}



inline int inv(int x){

	return fpow(x,Mod-2);

}



inline int C(int n,int m){

	return jc[n]*inv(jc[m])%Mod*inv(jc[n-m])%Mod;

}



inline int get(int x1,int y1,int x2,int y2){

	int x=x2-x1+1,y=y2-y1+1;

	return C(x+y-2,y-1);

}



signed main(){

	n=read();m=read();A=read();B=read();

	for(int i=1;i<N;i++) jc[i]=jc[i-1]*i%Mod;

	int x=n-A,y=B+1;

	while(x&&y<=m){

		ans=(ans+get(1,1,x,y)*get(x,y,n,m)%Mod)%Mod;

		x--;y++;

	}

	printf("%lld\n",ans);

	return 0;

}


// Faster C++ solution for the same problem (measured speedup: 1.9140768748359622x):
#include <bits/stdc++.h>

#pragma warning (disable:4996)



using namespace std;

using ll = long long;

using LL = ll;

using ull = unsigned long long;

using uLL = ull;

#define clean_0(A) memset((A),0,sizeof(A))

#define clean_1(A) memset((A),-1,sizeof(A))

#define For(i,a,b) for(int (i)=(a);(i)<=b;++(i))

#define Max_(a,b) ((a)>=(b)?(a):(b))

#define Min_(a,b) ((a)<=(b)?(a):(b))

template<typename T>

inline T Max(T a, T b) { return a >= b ? a : b; }

template<typename T>

inline T Min(T a, T b) { return a <= b ? a : b; }



constexpr int INF = 0x7F7F7F7F;

constexpr double eps = 1e-9;



constexpr ll mod1 = 197;

constexpr ll mod2 = 19260817;

constexpr ll mod3 = ll(1e8) + 7;

constexpr ll mod4 = ll(1e9) + 7;;

constexpr ll mod5 = 998244353;

constexpr int mod = mod4;



constexpr int maxn = int(3e5) + 1;

constexpr int MAXN = maxn;

namespace io {



#define in(a) (a=read())

#define out(a) write(a)

#define outn(a) (out(a),putchar('\n'))



#define I_int  int

	inline I_int read() {

		I_int x = 0, f = 1; char c = getchar();

		while (c < '0' || c > '9') { if (c == '-') f = -1; c = getchar(); }

		while (c >= '0' && c <= '9') { x = x * 10 + c - '0'; c = getchar(); }

		return x * f;

	}

	template<typename T>

	bool scan_d(T& ret)

	{

#define p(x) ((x)>='0'&&(x)<='9')

		int c; int sgn;

		if (c = getchar(), c == EOF)return false;

		while (c != '-' && !p(c))c = getchar();

		sgn = (c == '-') ? -1 : 1;

		ret = (c == '-') ? 0 : c - '0';

		while (c = getchar(), p(c))ret = ret * 10 + c - '0';

		ret *= sgn;

		return true;

#undef p

	}

	char F[200];

	template<typename T>

	inline void write(T x) {

		if (x == 0) { putchar('0'); return; }

		auto tmp = x > 0 ? x : -x;

		if (x < 0) putchar('-');

		int cnt = 0;

		while (tmp > 0) {

			F[cnt++] = tmp % 10 + '0';

			tmp /= 10;

		}

		while (cnt > 0) putchar(F[--cnt]);

	}

	void read(char* s, int Arr[], size_t n)

	{

		for (size_t i = 0; i < n; ++i)Arr[i] = strtoll(s, &s, 10);

	}

#undef I_int



}

using namespace io;





template<typename T>

T exgcd(T a, T b, T& x, T& y)

{

	if (a == 0 && b == 0)return -1;

	if (b == 0) { x = 1, y = 0; return a; }

	T d = exgcd(b, a % b, y, x);

	y -= a / b * x;

	return d;

}

//template<typename T>

//T inv(T a,T Mod=mod)

//{

//	T x, y;

//	return exgcd(a, Mod, x, y) == 1 ? (x % Mod + Mod) % Mod : -1;

//}

template<typename T>

constexpr T _gcd(T a, T b)

{

	return b ? _gcd(b, a % b) : a;

}

template<typename T>

constexpr T combinator(T n, T m, T Mod = mod - 1)

{

	m = min(m, n - m);

	T ret = 1;

	for (T i = 1; i <= m; ++i)

	{

		ret = ret * (n - m + i) / i % Mod;

	}

	return ret;

}

template<typename T>

constexpr T poww(T a, T b, T Mod = mod)

{

	T ret = 1;

	a %= Mod;

	b %= Mod - 1;

	if (b < 0)b += Mod - 1;

	while (b)

	{

		if (b & 1)ret = ret * a % Mod;

		a = a * a % Mod;

		b >>= 1;

	}

	return ret;

}



struct Combinator

{

	static constexpr int Mod = ::mod;

	static constexpr int Maxn = 1;//::maxn;

	int dp[Maxn][Maxn];//this is C(n,m)%(mod-1)

	constexpr Combinator() :dp()

	{

		for (int i = 0; i < Maxn; ++i)

		{

			for (int j = 0; j <= i; ++j)

			{

				if (j == 0) dp[i][j] = 1;

				else if (i == j) dp[i][j] = 1;

				else dp[i][j] = (dp[i - 1][j] + dp[i - 1][j - 1]) % (Mod - 1);

			}

		}

	}

};

//ll Fact[maxn], Inv[maxn];

//void Init(int n)

//{

//	Fact[0] = 1;

//	for (int i = 1; i <= n; i++)

//		Fact[i] = Fact[i - 1] * i % mod;

//	Inv[0] = 1;

//	Inv[n] = poww(Fact[n], mod - 2LL);

//	for (int i = n - 1; i > 0; i--)

//		Inv[i] = Inv[i + 1] * (i + 1LL) % mod;

//}

//ll C(int a, int b) {

//	if (a < 0 || b < 0)return 1;

//	return F[a] * Inv[b] % mod * Inv[a - b] % mod;

//}

//int main()

//{

//	Init(2.1e5);

//	int a, b, h, w;

//	while (cin >> h>>w>>a>>b)

//	{

//		ll ans = C(h + w - 2, h - 1);

//		for (ll i = 1; i <= b; i++) {

//			ans -= C(h - a + i - 2, h - a - 1) * C(a - 1 + w - i, a - 1) % mod;

//			ans = (ans % mod + mod) % mod;

//		}

//		printf("%lld", ans);

//	}

//}

int H, W, A, B;

const ll Maxn = maxn;

const int Mod = mod;

ll f[Maxn + 10], inv[Maxn + 10];

int N;

void Init() {

	f[0] = 1;

	for (int i = 1; i <= N; i++)

		f[i] = f[i - 1] * i % Mod;

	inv[0] = 1;

	inv[N] = poww(f[N], Mod - 2LL);

	for (int i = N - 1; i > 0; i--)

		inv[i] = inv[i + 1] * (i + 1) % Mod;

}

ll C(int a, int b) {

	if (a < 0 || b < 0)return 1;

	return f[a] * inv[b] % Mod * inv[a - b] % Mod;

}

int main() {

	scanf("%d %d %d %d", &H, &W, &A, &B);

	N = H + W - 2;

	Init();

	ll ans = C(H + W - 2, H - 1);

	for (ll i = 1; i <= B; i++) {

		ans -= C(H - A + i - 2, H - A - 1) * C(A - 1 + W - i, A - 1) % Mod;

		ans = (ans % Mod + Mod) % Mod;

	}

	printf("%lld", ans);

	return 0;

}