Why bitfields store negative number even after assigning a positive value?

Question

In the following code

#include<iostream>
using namespace std;
struct abc{
    int x:4;
} m;
int main(){
    m.x = 8;
    cout<<m.x;  
}

Here the variable x is signed and I assigned it a value of 8 but the output is coming out to be -8. If I change the declaration of int x:4 by unsigned int x:4 then the output is correct. Why the output in first case comes out to be -8 even though I assigned x a positive value? Please explain in detail. And also how are signed and unsigned variables stored in memory?