x440 is a "G1" platform, and so is supported with 15.x and 16.x EXOS version.
As Brad told you, 15.6.4 is the recommended EXOS release. You can find that information on that link:
http://documentation.extremenetworks.com/hw_sw_compatibility/hardwaresoftwarecompatibility/r_exos-re...
This link is updated every quarter (if I'm not mistaken). I believe you would prefer use the latest patch release of the recommended release (15.6.4.2p1-7).
The highest release you can install on a x440 is, currently, 16.1.3p1-2.