Not a bad idea at all. I had the same problem on my last MBA with only 4gb. Put Yosemite on and that was too much. Beachballs and slow responses on opening and closing apps. Memory pressure was very high. I pointed to the last Mavericks Time Machine file and reloaded back to where I had been. It worked fine with the reload, but there is no reason for it not to.
Files that you downloaded while using Yosemite may not work, however. Apps can usually go forward to a new OS, but many times not backwards to an old one. They can be expecting something that was not on the old OS. You would need to take your machine back to that last day of Mavericks and then reload the files into it from the source, not from a backup.
Apple is still selling MBA's with 4gb and Yosemite so they expect them to work, obviously. However, in Apple's defense, I can say that sometimes I try to do work on my MBA that really should be done on my iMac.
Even with Mavericks you should be good for a couple more years before new apps start complaining about the too-old OS.
Of course, any new machine should have at least 8gb (or 16 if available.)