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

using namespace std;

int fac(int k) {
    int res = k;
    for(int i = 0; i < k; i++) {
        res *= (k - 1);
        k--;
    }


    return res;
}

int main() {
    int a, b, c, k;
    long long int result = 0;


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

    if ( c != a && c != b )
    {
        cout << 0 << endl;
        return 0;
    }

    if ( a == b )
    {
        cout << k << endl;
        return 0;
    }

    long long tmpres = 0;

    for (int i = 0; i < k; i++) {
        tmpres += fac(k);
    }
    tmpres /= 2;

    //cout << tmpres % 1000000007 << endl;
    result = tmpres + k;



    /*if ( k != 1 )
        result += k * 2;
    else
        result += 1;

    long long 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 = ( tmp_result / 2 ) * i;
        result += tmp_result;
        //cout << tmp_result << ", " << result << endl;
    }
*/
    cout << result % 1000000007 << endl;
    return 0;
}