Duane–
Here is a great easy-to-access article on what Composite (vs. Base) radar is:
Essentially, the composite radar is the maximum signal at each radar angle. Base radar is the lowest angle (generally 0.5 degrees up). [Sadly I cannot find a simple explanation of 1km and 4km radar].
In other words, Composite radar is the worst precipitation in the column of air above a specific point.
–David