After you add the 4th control point, your table of points will start including error information, as the points are triangulated against one another. Note that this error may not mean that you are doing anything wrong, particularly in an older map that is not as spatially accurate as something more modern! On the other hand, if your error is quite high and you believe your map is relatively accurate, you may have misplaced a control point somewhere. Usually, high error is caused by a single point being misreferenced.