Sorry, I blocked that Mac.User kid. He needs a hug. But, none of these "updates" applied to me, which is why I asked.
Fixes an issue that may prevent Mail from displaying messages (never had a problem)
Improves AFP file transfer performance over 802.11ac Wi-Fi (don't have ac)
Resolves an issue that may prevent a screen saver from starting automatically (don't use screen savers)
Improves Xsan reliability (no idea what Xsan is)
Improves reliability when transferring large files over Ethernet (I don't transfer files over ethernet)
Improves performance when authenticating to an Open Directory server (no idea what an Open Directory server is, don't use it)
Addresses an issue that prevented a smart card from unlocking preference panes in System Preferences (don't know what a smart card is, don't use one)
Contains the improvements included in MacBook Air (Mid 2013) Software Update 1.0 (never installed this, mine may have come with it insalled, but I never suffered any of the problems with wi-fi/screen/etc. on my new Air)
For what it's worth, I decided to backup my Documents, Bookmarks and Downloads and went ahead with the update. Figured my computer is new enough and with those 3 things backed up I could go back to .4 easily if necessary. All went well, no problems, no noticeable difference as expected.