how to purchase applecare for iphone4 online

Do you already have the iPhone? If you do, you will need to go to your Apple Store and take your phone with you. Apple will not offer AppleCare to a product already in an owner's possesion without checking it out to ensure it is not damaged in any way. Also, I believe you only have 30 days post purchase to add AppleCare.

If you are ordering a phone from Apple online, you can add AppleCare at the time of the order.

There is ABSOLUTELY no way and you only had 30 days to purchase AppleCare from the day you bought your phone to begin with.

You have 1 year to purchase AppleCare. The 30 days limit is only if you want AppleCare + (plus) which covers accidental damage)