#include <bits/stdc++.h>
using namespace std;
 
typedef ostringstream OSS; typedef istringstream ISS;
typedef long long LL;
typedef pair<int, int> PII;  typedef pair<LL, LL> PLL;
typedef vector<int>    VI;   typedef vector<VI>   VVI;   typedef vector<VVI>   VVVI;
typedef vector<LL>     VLL;  typedef vector<VLL>  VVLL;  typedef vector<VVLL>  VVVLL;
typedef vector<double> VD;   typedef vector<VD>   VVD;   typedef vector<VVD>   VVVD;
typedef vector<string> VS;   typedef vector<VS>   VVS;   typedef vector<VVS>   VVVS;
typedef vector<bool>   VB;   typedef vector<VB>   VVB;   typedef vector<VVB>   VVVB;
typedef vector<PII>    VPII; typedef vector<VPII> VVPII; typedef vector<VVPII> VVVPII;
typedef vector<PLL>    VPLL; typedef vector<VPLL> VVPLL; typedef vector<VVPLL> VVVPLL;
 
typedef unsigned int  UI;
typedef vector<UI>    VUI;
typedef vector<VUI>   VVUI;
 
#define fst first
#define snd second
// #define Y first
// #define X second
#define MP make_pair
#define PB push_back
#define EB emplace_back 
#define ALL(x)  (x).begin(),(x).end()
#define RALL(x) (x).rbegin(),(x).rend()
#define RANGEBOX(x,y,maxX,maxY) (0 <= (x) && 0 <= (y) && (x) < (maxX) && (y) < (maxY))
#define RANGE(x, l, r) ((l) <= (x) && (x) <= (r))
#define rep(i, N)  for (int i = 0; i < (int)(N); i++)
#define rrep(i, N) for (int i = N - 1; i >= 0; i--)
#define REP(i, init, N)  for (int i = (init); i < (int)(N); i++)
#define RREP(i, N, last) for (int i = (init - 1); i >= last; i--)
#define MAXUD(orig, target) orig = max(orig, target)
#define MINUD(orig, target) orig = min(orig, target)
#define DUMP(x) cerr << #x << " = " << (x) << endl
 
template < typename T > inline T fromString(const string &s) { T res; ISS iss(s); iss >> res; return res; };
template < typename T > inline string toString(const T &a)   { OSS oss; oss << a; return oss.str(); };
 
template<typename T=int> inline void dump(vector<T> vs, bool ent=false) { rep(i, vs.size()) cout << vs[i] << (i+1==vs.size() ? '\n' : ' '); if (ent) cout << endl; }
template<typename T = int> inline void dump(vector<vector<T>> vs, bool ent=false) { rep(i, vs.size()) dump<T>(vs[i]); if (ent) cout << endl; }
 
const int    INF  = 0x3f3f3f3f;
const LL     INFL = 0x3f3f3f3f3f3f3f3fLL;
const double DINF = 0x3f3f3f3f;
const int    DX[] = {1,  0, -1, 0};
const int    DY[] = {0, -1,  0, 1};
const double EPS  = 1e-12;
// const double PI   = acos(-1.0);
 
template<typename T = int> inline T in() { T x; cin >> x; return x; }
template<typename T = int> inline vector<T> in(int n) { vector<T> xs(n); rep(i, n) cin >> xs[i]; return xs; }
template<typename T = int> inline vector<vector<T>> in(int n, int m) { vector<vector<T>> xs(n, vector<T>(m)); rep(i, n) rep(j, m) cin >> xs[i][j]; return xs; }
 
const LL MOD = 1000 * 1000 * 1000 + 7;
 
LL power_mod(LL n, LL p) { // n^p
    VLL memo(64);
    memo[0] = n;
    REP(i, 1, 64) {
        memo[i] = memo[i - 1] * memo[i - 1] % MOD;
    }

    LL res = 1;
    for (LL i = 0; i < 64; i++) {
        if (p >> i & 1) {
            (res *= memo[i]) %= MOD;
        }
    }

    return res;
}
 
 
// x^(-1) (mod MOD) xの逆元を求める
LL inverse(LL x) {
    return power_mod(x, MOD - 2);
}
 
// stadk overflow 注意
LL factorial(LL n) { // n!
    static map<LL, LL> memo;
    if (!n) return 1;
    if (n <= 2) return n;
    if (memo.count(n)) return memo[n];
    return memo[n] = (n * factorial(n - 1)) % MOD;
}
 
LL sub_factorial(LL n, LL m) { // n * (n + 1) * ... * (m - 1) * m
    LL a = factorial(m);
    LL b = factorial(n - 1);
    LL invb = inverse(b);
    return (a * invb) % MOD;
}
 
 
LL comb_mod(LL n, LL r) { // nCr
    static map<pair<LL, LL>, LL> memo;
 
    if (n - r < r) {
        return comb_mod(n, n - r);
    }
 
    pair<LL, LL> key(n, r);
 
    if (memo.count(key)) {
        return memo[key];
    }
 
    LL pn = sub_factorial(n - r + 1, n);
 
    LL pr = factorial(r);
 
    return memo[key] = pn * power_mod(pr, MOD - 2) % MOD;
}
 
int main(void) {
    int H = in(), W = in(), A = in(), B = in();
    --H; --W;
 
    LL a = comb_mod(H + W, H);
    rep(i, B) {
        LL b = comb_mod(H - A + i, i) *
            comb_mod(A - 1 + W - i, A - 1) % MOD;
        a = (a - b + MOD) % MOD;
    }
 
    cout << a << endl;
 
    return 0;
}


Measured memory_bytes: 37760