The window.open() function accepts the name of the window as the second parameter.
window.open(url, name);
When you already have a popup window with the same name open, your browser will reuse that window.
So for example
window.open('/Page1.aspx', 'SameWindow');
window.open('/Page2.aspx', 'SameWindow');
window.open('/Page3.aspx', 'SameWindow');
window.open('/Page4.aspx', 'SameWindow');
These will all open in the same window.