A key element in music production is consistency. Working 4 hours in a day and then taking a break of 2 days, is not being consistent.
Instead you want to set up a time for ever day. For instance in my case, I produce about 90 minutes a day with a break after 45 minutes. Within these 90 minutes I put up a small goal. I might start with melody & chords and a basic sound idea. This is my main goal for first 45 minutes. After the break I will arrange it into my pre-made arrangement. When the 90 minutes are over, I'm done. I don't produce longer, nor shorter. If shorter, I organize what I made to keep me busy.
After building up consistency, goals are another important subject. Before you start you should set up a goal for your session. Make it a small one. Don't do more. Sure you can do more and work longer. But you create a higher chance to fudge* everything up. With smaller and less goals per session, you'll gain more satisfaction and motivation. Simply because of the fact you accomplished something today.
I hope this helps you a little bit. It's all a mind game, which can be easily overcome with small steps.