Q: How do I determine how many GB my iPhone 4 is?

If the capacity is 28.4 GB, then you have a 32GB phone.

Based on your numbers, you have a 32GB iphone.  The way memory size is determined can be read in HT1867

http://support.apple.com/kb/HT1867.  32 GB is the actual storage size, but the operating system takes up a portion of this.  Read the artice for more detail.

HTH

Interesting.  Duplicate posts.  Both articles are relevant.

Message was edited by: ssschmidt

TS2419 explains how memory is calculated.  Some of the memory is not accessible to the user because it stores the operating system files.

http://support.apple.com/kb/TS2419