For sure pic #1 is a RDS cover for a Docter sight. I've seen that come with some ACM docter sights.
#2/3 is the rear tube adaptor thing for the Magpul UBR stock. Not sure how it works but I'm told that it replaces the M4 buffer tube. If you see a picture of the UBR stock you'll notice that it's "smaller" than the standard M4 buffer tube. Here's a pic of the whole set: http://www.airsofthunting.com/images...ck%20TAN-1.jpg
#4 If it's SystemA it should be 6.04, not sure if they sell any other bore sizes but 6.04 is the only one I've seen and TBH it's pretty good since SystemA stuff is pretty consistent and high quality (barrel consistency as well as bore size affect the final outcome not just tightness of the bore). eg. Tight barrel that's kinked will give you weird trajectories whereas a "looser" barrel that's consistent will give you a consistent trajectory. That is a 6.04 Prometheus barrel will probably be better than a 6.01 cheapo ACM barrel.