int f1(int, int, int, int);
int f2(int, int, int, int);
int f3(int x1, int x2, int x3, int x4);
int f4(int x1, int x2, int x3, int x4);

int f2(int y1, int y2, int y3, int y4) {}
int f4(int y1, int y2, int y3, int y4) {}