selectedcount = 0;

function CompareAddRemove(string, obj) {

    if ( obj.checked ) {
        if ( selectedcount > 3 ) {
            alert('En fazla 4 tane ekleyebilirsiniz');
            obj.checked=false;
            return false;
        }
        selectedcount++;
    } else
        selectedcount--;

    var seladdremove = document.getElementById('seladdremove');

    seladdremove.src = 'seladdremove.php?script=&s'+obj.checked+'=' + string;

    return true;
}