Don’t worry too much yet.
At 4 months, this is fairly common-babies can be distracted and give uneven hearing test results. That’s why your doctor planned a repeat test at 6 months.
Cotton buds usually don’t cause hearing damage unless inserted deep, so it’s unlikely you caused this.
> Most babies who fail one side initially pass the follow-up test.
Just wait for the recheck and observe if she responds to sounds at home