Here's your code indented and fixed up a bit.
#include <iostream>
#include <string>
using namespace std;
int main(void)
{
system("TITLE Addition");
system("COLOR 3");
int number1 = 0, number2 = 0, number3 = 0, number4 = 0, sum = 0;
// int one = 1; <-- never used
string yesno;
// string relay; <-- useless
bool bCont = true;
while (bCont) {
cout<<"I'm going to add these numbers you give me. First Number: ";
cin>>number1;
cout<<"next number: ";
cin>>number2;
cout<<"and the next number: ";
cin>>number3;
cout<<"and the last number: ";
cin>>number4;
sum = number1 + number2 + number3 + number4;
if(sum < 10)
cout<<"Your number is " << sum << " and it is a small number. \n";
else if ( sum > 10 && sum < 50 )
cout<<"Your number is " << sum << " and it is a big number. \n";
else if (sum > 50 && sum <150 )
cout<<"Your number is " << sum << " and it is a large number. \n";
else
cout<<"You have a really big number \n" ;
cout<<"Try again? Y/N: ";
cin>> yesno;
if (yesno == "Y" || yesno == "y")
continue;
else
bCont = false;
}
cin.get();
return 0;
}