The KA M4 is difficult to reassemble. Something in the tolerances in this gun makes sliding the upper and lower back together very difficult. In addition, the faux bolt will cause you a lot of grief when you are putting this gun back together. I'm not gonna lie, you have a bit of learning ahead of you.
Just make sure to go slow and don't put a ton of pressure on anything. If it requires an immense amount of pressure, there's probably a better way.