Is your iphone 4 unlocked? If so you just take a sim card from LG optimus and put it into the iphone if sim card is same size.
If it uses a different size sim card, you just need a prepaid micro sim card.
Then go to Self serve > Mobile Phone > Swap Mobile Phone
Step 1 : "Upgrade my current phone"
Step 2 : "Does the new phone you're upgrading to use a SIM card?"
Select "yes", and enter a new sim # then continue