Well.. it's not a case of 'settling' for an answer.. What Blueberry said is correct..
Its a fact.
I'll also add to that two things..
1) OS X needs roughly 20% of system drive free space to work efficiently.. It needs 'breathing space' if you like,. and if you are running OS X with less than 20% of free system drive space then OS X's performance will become sluggish...
2) When you do anything in OS X some disk space is often used temporarily in place of system memory and that alone can quickly 'eat' into what you think you have as free space.. so add to that the act of downloading 4GB that can and will actually require much more space than simply what is needed by the download itself.. and you run out of your available space real quick. This is what Blueberry is aluding to...
As an aside...A Macbook Air with their typically small system drives.. are great for what they are designed to do.. but running a memory and HD intensive program such as Logic Pro.. is not really their forte by any means,.. They can do it of course, but you have to make many compromises in the process.. and one of those includes using external HDs for storage of anything you don't need on a daily basis so you keep your System drive as 'clean' as possible .. and that means sacrificing convenience..