Mission 4

Here's a level that can be completed swiftly.  As soon as the level starts (not a second too late!), fire all 5 of your rounds successively, starting with the first aimed at the upper left brown tank, ending with the last at the lower right brown tank, with the other rounds in between.  If you're lucky, that's all it takes, taking out all 4 tanks in one blow.  If not, that will usually take out 2 or 3 tanks right away.

With or without this tactic, the main thing to pay attention to in this level is evading enemy fire by moving to the space in the "V" of the trajectory of enemy fire.   This is because you already start out near the corner and you don't want to be further cornered!