Reinstall Yosemite on Macbook Air
From OS X: About OS X Recovery


  Which version of OS X is installed by OS X Recovery?
  
  
  If you use the Recovery System stored on your startup drive to    reinstall OS X, it installs the most recent version of OS X previously installed on this computer.
  If you use Internet Recovery to reinstall OS X, it installs the version of OS X that originally came with your computer. After installation is finished, use the Mac App Store to install related updates or later versions of OS X that you have previously purchased.
  


So as long as the local Recovery disk partition is intact it will install, at this point in time, OS X Yosemite 10.10.4.