Q: When I add up usage on my phone I see approx 4gb used and yet I only have 900 mb avail.  Help!

Hello Noradora,

After reviewing your post, it sounds like you are running low on space, but the storage doesnt add up correctly. I would recommend that you read this article, it may be able to help the issue.

In iTunes there are things like "Other: Settings, Siri voices, and other system data" that dont show up in the storage menu. Check the storage in iTunes. 

See how much storage you've used on your iPhone, iPad, and iPod touch - Apple Support

Thanks for using Apple Support Communities.

Have a nice day,

Mario

Hey Noradora,

Since the storage is filling up and not consistent with iTunes. I would recommend that you read this article, create a backup of the device first. Once it is restored the data should match. You can restore your data back to the iPhone. 

Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Thanks for using Apple Support Communities.

Have a good one,

Mario