Do not have Personal Hotspot
Hello, I've just bought an iPhone 4 and it didn't have personal hotspot.
iPhone 4, iOS 5.1.1
Verify that your carrier supports Personal Hotspot for your device and that you have Personal Hotspot enabled on your cellular plan:
From Here > http://support.apple.com/kb/HT3574
Personal Hot spot
iOS System Requirements > http://support.apple.com/kb/HT3574
Understanding > http://support.apple.com/kb/HT4517
Trouble Shooting > http://support.apple.com/kb/TS2756