#include <iostream>
#include <cstdlib>
#include <cmath>

using namespace std;

int main() {
    int a, b, c, k;
    int result = 0;
    bool ac = false, bc = false;

    cin >> a >> b >> k >> c;

    if (a == c) {
        ac = true;
    } else if (b == c) {
        bc = true;
    }

    result += k * 2;

    int tmp_result;

    for (int i = 2; i < k; i++) {
        tmp_result = 1;
        for ( int braces = 0 ; braces < i ; braces++ )
            tmp_result *= k - braces;

        tmp_result /= 2;
        result += tmp_result * i;
    }

    //cout << a << b << k << c << ac << bc << endl;

    cout << result % 1000000007 << endl;


    return 0;
}