This isn't anything to be concerned about given you are on estrogen and have had a negative biopsy.
The normative cut off valve for endometrial thickness of 4mm was derived from untreated postmenopausal women.
No suitable cut off value for women on HRT has been agreed upon at this time by relevant professional bodies but different experts have postulated values from 8mm to 11mm.
If you or your GP are worried you can arrange a follow up scan in 6 months or so, with or without a course of oral provera.