Most likely you have your machine set to constant velocity instead of exact stop. M4 will round the corners to keep the velocity. You can test this by making several squares and give each one a different velocity. This will allow you to see how M4 rounds the corners to keep the constant velocity. I ramped up the feed rate for each square by 10 ipm to see how it worked on my machine. You can either change the feed rate to something that your configuration of M4 likes better, change your motor settings (e.g., velocity, acceleration), or change M4 to exact stop.
I suggest you play with the settings to see what works best on your machine,
P