Mobile hotspot missing on iphone 4
If you are you on a carrier that is officially supported by Apple, your need a mobile contract that allows Personal Hotspot, otherwise it will not work. If your carrier allows it, update your Carrier Settings:


Make sure internet works
Settings > General > About
You will see a popup to inform you of the update.


If you are on an unofficial carrier, you should configure Personal Hotspot manually: 


Settings > Cellular > Cellular Data Network
Scroll to the bottom, under Personal Hotspot, enter the APN, Username and Password you received from your carrier.


If any of the above options are missing, it means that your carrier has disabled them.