#include<cstdio>

#define LL long long
#define MAXN 100000
#define MOD 1000000007
LL fac[2*MAXN+5];
LL inv[2*MAXN+5];

LL Pow(LL x,LL p){
    LL ret=1;
    while(p){
        if(p&1)
            ret=ret*x%MOD;
        x=(x*x)%MOD;
        p>>=1;
    }
    return ret;
}

void Prep(int N){
    fac[0]=1;
    for(int i=1;i<=N;i++)
        fac[i]=fac[i-1]*i%MOD;
    inv[0]=1;
    inv[N]=Pow(fac[N],MOD-2);
    for(int i=N-1;i>=1;i--)
        inv[i]=inv[i+1]*(i+1)%MOD;
}

LL C(int m,int n){
    return fac[m]*inv[n]%MOD*inv[m-n]%MOD;
}

int main(){
    int H,W,A,B;
    scanf("%d%d%d%d",&H,&W,&A,&B);
    Prep(H+W);
    LL Ans=0;
    for(int i=B+1;i<=W;i++){
        LL tmp1=C(H-A+i-2,i-1);
        LL tmp2=C(A+W-i-1,W-i);
        Ans=(Ans+(tmp1*tmp2)%MOD)%MOD;
    }
    printf("%lld",Ans);
}
