using System;
using System.IO;

#region Program
namespace Program
{
    #region Solver
    public class Solver
    {
        static IO.StreamScanner
        sc = new IO.StreamScanner(Console.OpenStandardInput());

        static readonly int INF = 1000000007;

        int[] HWAB = new int[4];
        int[] FCT;
        int[] INV;

        int H;
        int W;
        int A;
        int B;

        int BIG;

        #region Solver
        public Solver()
        {
            HWAB = sc.Integer(4);
            H = HWAB[0];
            W = HWAB[1];
            A = HWAB[2];
            B = HWAB[3];
            BIG = H <= W ? W : H;

            // 階乗テーブル作成
            factorial(H + W - 2);

            // 逆元テーブル作成
            inverse(BIG);
        }
        #endregion

        #region Solve
        public void Solve()
        {
            int top = 0;
            int bottom = 0;
            int ans = 0;

            int ta = H - A - 1;
            int ba = A - 1;

            for (int i = B; i < W; i++)
            {
                // a+bCa
                // a = H - A - 1
                // b = i
                top = combination(ta, i);

                // a+bCa
                // a = A - 1
                // b = W - 1 - i
                bottom = combination(ba, W - 1 - i);

                // 排反事象は、足し算
                // 連続試行は、掛け算
                ans = modadd(ans, modmulti(top, bottom));
            }

            Console.WriteLine(ans);
        }
        #endregion

        #region combination 組み合わせ(a+bCa)
        int combination(int a, int b)
        {
            int tmp = modmulti(FCT[a + b], INV[a]);
            return modmulti(tmp, INV[b]);
        }
        #endregion

        #region factorial 階乗テーブル
        void factorial(int x)
        {
            FCT = new int[x + 1];
            FCT[0] = 1;
            FCT[1] = 1;
            for (int i = 2; i <= x; i++)
            {
                FCT[i] = modmulti(FCT[i - 1], i);
            }
        }
        #endregion

        #region inverse 逆元テーブル
        void inverse(int x)
        {
            INV = new int[x + 1];
            INV[0] = 1;
            INV[1] = 1;
            INV[x] = modcalc(FCT[x], INF - 2);
            for (int i = x - 1; 2 <= i; i--)
            {
                INV[i] = modmulti(INV[i + 1], i + 1);
            }
        }
        #endregion

        #region modadd aとbを足した値をmodする(a + b mod p)
        int modadd(int a, int b)
        {
            return (a + b) % INF;
        }
        #endregion

        #region modmulti aとbを掛けた値をmodする(a * b mod p)
        int modmulti(int a, int b)
        {
            int res = 0;
            int mod = a % INF;
            while (b > 0)
            {
                if ((b & 1) == 1)
                {
                    res += mod;
                    if (res > INF)
                    {
                        res -= INF;
                    }
                }
                mod <<= 1;
                if (mod > INF)
                {
                    mod -= INF;
                }
                b >>= 1;
            }
            return res;
        }
        #endregion

        #region modcalc aのb乗をpで割った余りを計算する(a ^ b mod p)
        int modcalc(int a, int b)
        {
            // O(log b)
            int d = 0;
            if (b == 0)
            {
                return 1;
            }
            else if ((b & 1) == 0)
            {
                // 偶数 bを半分にできる
                d = modcalc(a, b / 2);
                return modmulti(d, d);
            }
            else
            {
                // 奇数 -1乗から1乗をかける
                return modmulti(a, modcalc(a, b - 1));
            }
        }
        #endregion
    }
    #endregion
}
#endregion

#region Execute
class Execute
{
    public static void Main(string[] args)
    {
        new Program.Solver().Solve();
        Program.IO.Printer.Out.Flush();
    }
}
#endregion

#region Program.IO
namespace Program.IO
{
    using System.Text;
    using System.Globalization;
    using System.Linq;
    public class Printer : StreamWriter
    {
        static Printer()
        {
            Out = new Printer(Console.OpenStandardOutput())
            {
                AutoFlush = false
            };
        }
        public static Printer Out
        {
            get;
            set;
        }
        public override IFormatProvider FormatProvider
        {
            get
            {
                return CultureInfo.InvariantCulture;
            }
        }
        public Printer(System.IO.Stream stream)
            : base(stream, new UTF8Encoding(false, true))
        {
        }
        public Printer(System.IO.Stream stream, Encoding encoding)
            : base(stream, encoding)
        {
        }
        public void Write<T>(string format, T[] source)
        {
            base.Write(format, source.OfType<object>().ToArray());
        }
        public void WriteLine<T>(string format, T[] source)
        {
            base.WriteLine(format, source.OfType<object>().ToArray());
        }
    }
    public class StreamScanner
    {
        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof = false;
        public StreamScanner(Stream stream)
        {
            str = stream;
        }
        private T[] enumerate<T>(int n, Func<T> f)
        {
            var a = new T[n];
            for (int i = 0; i < n; ++i) a[i] = f();
            return a;
        }
        public long Long()
        {
            if (isEof)
            {
                return long.MinValue;
            }
            long ret = 0;
            byte b = 0;
            var ng = false;
            do
            {
                b = read();
            }
            while (b != '-' && (b < '0' || '9' < b));
            if (b == '-')
            {
                ng = true;
                b = read();
            }
            for (; true; b = read())
            {
                if (b < '0' || '9' < b)
                {
                    return ng ? -ret : ret;
                }
                else ret = ret * 10 + b - '0';
            }
        }
        public long[] Long(int n)
        {
            return enumerate(n, Long);
        }
        public int Integer()
        {
            return (isEof) ? int.MinValue : (int)Long();
        }
        public int[] Integer(int n)
        {
            return enumerate(n, Integer);
        }
        public char[] Char(int n)
        {
            return enumerate(n, Char);
        }
        public char Char()
        {
            byte b = 0;
            do
            {
                b = read();
            }
            while ((b < 33 || 126 < b) && !isEof);
            return (char)b;
        }
        public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
            {
                sb.Append(b);
            }
            return sb.ToString();
        }
        private byte read()
        {
            if (isEof)
            {
                return 0;
            }
            if (ptr >= len)
            {
                ptr = 0;
                if ((len = str.Read(buf, 0, 1024)) <= 0)
                {
                    isEof = true;
                    return 0;
                }
            }
            return buf[ptr++];
        }
    }
}
#endregion


Measured memory_bytes: 3928