Is it fine to update my phone to 9.3.2 iOS if I'm using iPhone 4s
You didn't indicate if your iPhone is jailbroken or not so I'm going on the assumption that it's not.

My recommendation to all questions like this is to say 'yes, you should always upgrade to the most recent iOS version'.  However, only you can decide if it's worth the upgrade or not.  If you do decide to upgrade, be sure to do a backup first.  If you don't like it, you can go back to what you have.

There are not many general user enhancements in this version (here is the list of changes), but there are many security and bug fixes that you should have (as documented here).  Most of the issues being reported in this version reference the iPad Pro.  If you have an iPad Pro, I recommend not upgrading right now.