This is extremely likely to just be a benign functional cyst following a last ovulation.
The 12 months cut off for considering bleeding "postmenopausal" is completely arbitrary, historically defined by a committee of gynaecologists who would probably have been all or mostly male given the era.
They could quite easily have settled on 10 months, or 18 months for example.
A 4mm endometrium is also not concerning especially in someone with evidence of recent ovulation and I personally would have simply elected to monitor this with another scan in 3-6 months, however if you are worried the biopsy should give you peace of mind.
It is a good idea to get a CA-125 blood test as well, however it really doesn't sound like anything sinister from what you have described.
I would also take this as an opportunity to think about your overall health going forward and whether you want to replace the ovarian hormones that your body is no longer reliably producing.