In my lab, we've found that the best cardio to build and maintain muscle mass is high-intensity interval sprints. Have you ever seen the hamstrings and quads of an Olympic sprinter? Then you know exactly how that type of movement can actually boost your body's ability to increase mass.
I want you to do at least one day of cardio per microcycle during Project Mass, but that doesn't have to be today. You'll have another chance after the next three lifting days.
If you're completely spent from the last three days of heavy training, take the day off. Allow your body to recover from the work, and repair for tomorrow. This trainer is not easy, so you'll have to learn how to listen to your body. As you get stronger, you'll be able to handle more work. When you start to feel more conditioned, you can up the intensity of your weekly cardio session or knock out two sessions per week.
If you choose to take a rest day, you don't need to include the post-workout meal in your nutrition plan.