I just bought a 2014 Macbook Air, it's only got a 1.4 GHz CPU and a very sad 4GB of RAM. ...what are the best things I can do to speed this thing up...
I have several MBA's in my office that have the same amount of RAM, and the same or worse specs and they are all older than your MBA. They aren't
at all slow, and 4GB of RAM is
not at all "sad" for just about all purposes.
The best thing that you can do to speed your MBA up is upgrade to the most recent operating system version that makes sense. With every operating system upgrade Apple has included more advanced memory management and compression, and has made things run a bit faster. My experience, though, is that you won't likely realize the latter unless you do a clean install of the new OS.
Your MBA can be upgraded all the way up to the latest version of the Mac OS, which is macOS 10.15 (Catalina)....which I don't recommend because it won't run 32-bit apps and it still likely has a data-loss problem in Mail.
So, if your MBA doesn't already have Mojave installed, that's what I recommend:
MacOS 10.14 - Mojave - FREE
Connecting to the Mac App Store
Let me know if you have problems downloading or installing Mojave...some folks have been having an expired certificate problem that requires a work-around.
Here is how to do a clean install of the Mac OS:
Upgrading To The Very Latest Macintosh Operating System
Especially item #6
I'm assuming that your MBA is already running just fine. If it is currently running slow, then it has a problem, and you should fix that first. See:
Macintosh Slowdown Solutions
Macintosh OS X Slowdown Solutions