You should weigh your options with all the junk in the garage and determine what is more important to you, those boxes that have stuff you havent touched or looked at in over 4 years or the well being of your bikes from having made the right decision by making room for them in the garage rather than letting them rust out in the backyard so that the next time you need them they are all nice and rusted.....Not a hard one to figure. Besides that if the bikes you have will rust by being left outside then you might want to consider an upgrade since only low end bikes tend to have parts that can rust in the first place. Put them in the garage or spend more money anyway!
Anything you use to cover them outside to keep rain off will capture condensation underneath at night which will defeat the whole purpose of covering them in the first place and will lead to rust- dont believe me? Go right ahead and do it and I will know you did when you come back and ask if it is ok to use WD 40 to clean a rusty chain later on.....
