I cannot seem to beat the level Four Bosses in Blue Defense. Any tips or techniques?
try focusing fire on one of them as long as you can before taking down the streamers from the other three.
you should be able to fully kill one of them before worrying about the others.
aside from that, make sure that when you're not shooting down the streamers, that you focus fire on a single boss. if you spread your fire between them equally, then they will fire too many streamers at you, and you won't be able to kill them in time.
final tip: kill bosses 1 and 2 first, or 3 and 4 - if you kill two on the same end of the playing field first, you'll spend less time shooting into thin air on the long sides of the play field.
1 --- 2
3 --- 4
I hope this helps.
If you've got the full version, practice this strategy a few times in level select mode before going back to the regular game.