If I play Hard Bargain repeatedly, can the enemy use Defy? I paid 4.

Yes, your opponent can use Defy to counter your Hard Bargain, even if you paid the additional cost to Repeat it.

According to FAQ #256 and FAQ #9208, here is how this interaction works:

1. Repeat is an Additional Cost: You must choose whether to pay the Repeat cost when you play Hard Bargain from your hand, before it is finalized onto the chain.
2. Single Entity on the Chain: Once finalized, the spell (including the repeated portion) exists as a single entity on the chain. It is not two separate spells.
3. Defy Targetability: Defy checks the printed (base) cost of the target spell. Hard Bargain has a printed cost of [2] Energy and no Power, which makes it a valid target for Defy (which can counter spells with a base cost of 4 or less Energy and 1 or less Power).
4. Full Counter: Because Hard Bargain is a single entity on the chain, Defy counters the entire spell, including the repeated portion. None of the effects occur, and you do not get a refund for the costs you paid (per Rule 412.1.c).

In summary, paying for Repeat makes the spell more powerful if it resolves, but it does not change the spell's printed cost, nor does it create a second spell that is immune to being countered by Defy.