I have no idea what network design you have.  In our network, we are happy upgrade the firmware as often as possible.

There are two schools-of-thought.  One is "If it ain't broke, don't fix it" and another is "Upgrade at your own peril".  I'm the latter.

Cisco recommends anyone stay away from 5.X (it's a lost-cause).  The safest 6.X is 6.0.199 and the latest version of 7.X is also good.  (I'm running 7.X).

Before you make a decision to upgrade, make sure you read the Release Notes.  The Release Notes will tell you possible bugs you might run into with the new IOS.  And because you have 4.X, there's an upgrade path you need to consider.

Hope this helps.  Please don't forget to rate useful posts.