Every time you use your Mounjaro pen, you need to prime the pen to remove any air bubbles. As part of the priming process, you will lose some liquid and therefore the manufacturers have included extra liquid in addition to the four doses to make sure you don't lose any medication for treatment. This means that after you've used the pen for your four treatments, you may still have some liquid left in your pen.
It will not be a full dose in volume and you will not be able to administer it - this is expected, is purely to ensure that your four treatment doses are full and complete, and that priming the pen does not impact the treatment dose volumes. You can dispose of the pen once you've used all four treatments, even if there is still some liquid remaining.