Level 4 is just more of the same from level 2, and is probably the least interesting level in the game. The only extra in level 4 is the protected targets which are gold squares until you break open the seal revealing the target behind it. Overall it's tougher than level 2.

The boss in level 4 is a wall with a turret, enemies attacking from each side, and 3 targets. Kill the enemies that come out, as you focus on the turret. When the turret is gone, take out the targets, and two translucent heads appear at the top, which look like little transformer logos. They shoot out annoying bubbles that will take some practice getting used to. Refer to the tricks of the trade section for a detailed strategy on how to beat them.