I have always found it difficult to identify where the 4th or 5th intercostal space is located by counting the ribs.  After performing thousands of cases,  I have figured out the most simplistic and accurate way to make your incision is to use the mid portion of the sternum as your external landmark. That means that I mark the mid way point from the supra sternal notch to the lowest portion of the xiphoid.  I start my incision exactly at this point at the anterior axillary line and curve it superiorly.In 98 % of the cases this will be exactly where you will find the best visibility of the mitral valve. I have enclosed a picture of a male patient. Same holds true for a female. I will post pictures of female patients later this weekend with different options to consider.

Sorry but the OR is calling.