#include <bits/stdc++.h>
#include <unordered_map>

using namespace std;
typedef long long int ll;
typedef long double ld;
#define rep(i, n) for(ll i = 0; i < (ll)(n); i++)
#define repr(i, n) for (ll i = (ll)(n) - 1; i >= 0; i--)
#define all(x) (x).begin(),(x).end()
#define MOD 1000000007
#define INF (1LL << 62)

#define PI (acos(-1))
#define print(x) cout << x << endl
 
ll gcd(ll a, ll b) { return b ? gcd(b,a%b) : a;}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll countDigit(ll n) { return floor(log10(n) + 1); } 
typedef pair <ll,ll> P;
static const ll dy[4] = {0,1,0,-1}, dx[4] = {1,0,-1,0};

template <typename T> 
ostream& operator<<(ostream& os, const vector<T>& v) 
{ 
    for (int i = 0; i < (int)v.size(); ++i) os << v[i] << " ";  
    return os; 
}

template <typename T1, typename T2> 
ostream& operator<<(ostream& os, const map<T1, T2>& m) 
{ 
    for (auto p : m) os << "<" << p.first << ", " << p.second << "> "; 
    return os; 
}


template<int mod>
struct ModInt{
    int x;
    ModInt():x(0){}
    ModInt(long long y):x(y>=0?y%mod:(mod-(-y)%mod)%mod){}
    ModInt &operator+=(const ModInt &p){
        if((x+=p.x)>=mod)x-=mod;
        return *this;
    }
    ModInt &operator-=(const ModInt &p){
        if((x+=mod-p.x)>=mod)x-=mod;
        return *this;
    }
    ModInt &operator*=(const ModInt &p){
        x=(int)(1LL*x*p.x%mod);
        return *this;
    }
    ModInt &operator/=(const ModInt &p){
        *this*=p.inverse();
        return *this;
    }
    ModInt &operator^=(long long p){
        ModInt res = 1;
        for (; p; p >>= 1) {
            if (p & 1) res *= *this;
            *this *= *this;
        }
        return *this = res;
    }
    ModInt operator-()const{return ModInt(-x);}
    ModInt operator+(const ModInt &p)const{return ModInt(*this)+=p;}
    ModInt operator-(const ModInt &p)const{return ModInt(*this)-=p;}
    ModInt operator*(const ModInt &p)const{return ModInt(*this)*=p;}
    ModInt operator/(const ModInt &p)const{return ModInt(*this)/=p;}
    ModInt operator^(long long p)const{return ModInt(*this)^=p;}
    bool operator==(const ModInt &p)const{return x==p.x;}
    bool operator!=(const ModInt &p)const{return x!=p.x;}
    explicit operator int() const { return x; }						   // added by QCFium
    ModInt operator=(const int p) {x = p; return ModInt(*this);} // added by QCFium
    ModInt inverse()const{
        int a=x,b=mod,u=1,v=0,t;
        while(b>0){
            t=a/b;
            a-=t*b;
            std::swap(a,b);
            u-=t*v;
            std::swap(u,v);
        }
        return ModInt(u);
    }
    friend std::ostream &operator<<(std::ostream &os,const ModInt<mod> &p){
        return os<<p.x;
    }
    friend std::istream &operator>>(std::istream &is,ModInt<mod> &a){
        long long x;
        is>>x;
        a=ModInt<mod>(x);
        return (is);
    }
};
typedef ModInt<MOD> mint;
 
struct MComb {
    std::vector<mint> fact;
    std::vector<mint> inversed;
    MComb(int n) { // O(n+log(mod))
        fact = std::vector<mint>(n+1,1);
        for (int i = 1; i <= n; i++) fact[i] = fact[i-1]*mint(i);
        inversed = std::vector<mint>(n+1);
        inversed[n] = fact[n] ^ (MOD-2);
        for (int i = n - 1; i >= 0; i--) inversed[i]=inversed[i+1]*mint(i+1);
    }
    mint ncr(int n, int r) {
        return (fact[n] * inversed[r] * inversed[n-r]);
    }
    mint npr(int n, int r) {
        return (fact[n] * inversed[n-r]);
    }
    mint nhr(int n, int r) {
        assert(n+r-1 < (int)fact.size());
        return ncr(n+r-1, r);
    }
};
 
mint ncr(int n, int r) {
    mint res = 1;
    for (int i = n - r + 1; i <= n; i++) res *= i;
    for (int i = 1; i <= r; i++) res /= i;
    return res;
}

int main()
{
    ll H, W, A, B;
    cin >> H >> W >> A >> B;
    MComb mc(H+W);
    mint ans = mc.ncr(H+W-2, H-1);
    rep(i, A) {
        ans -= mc.ncr(H-A+i+B-1, B-1) * mc.ncr(W-B-1+A-1-i, W-B-1);
    }
    print(ans);
    return 0;
}

Measured memory_bytes: 1792