Inserting addresses into sets, the size of the sets is smaller than expected

I am getting a size of 1. Shouldn't it be 4? I am inserting the addresses of integers into the sets.

void func(set<int*>& s1, set<int*>& s2, int a)
{
    s1.insert(&a);
    s2.insert(&a);
}

int main()
{
    set<int*> s1, s2;

    int a = 1, b = 2, c = 3, d = 4;
    func(s1, s2, a);
    func(s1, s2, b);
    func(s1, s2, c);
    func(s1, s2, d);

    cout<<"  s1.size = "<<s1.size()<<"  s2.size = "<<s2.size()<<endl;
}