Hi alibabs1965

If you haven't had a period for 4 years ( have you been on HRT though?) then it could be that your womb lining was very thin so the initial bleeding and spotting many women experience is happening a bit later. The other alternative is that the endometrial lining has atrophied as an over-thin lining can also cause bleeding - not sure whether this would be bright red blood though, like a period, or spotting?
Did you have fibroids or anything like that when you were menstruating?
Personally in your position I would actually consult your doctor - just to get checked out and to be on the safe side. It could just be that the oestrogen is a bit too high for the progestogen in your case - leading to a build up of lining. If this turned out to be the case then a change in HRT would ensure your lining was protected. Whatever it is - maybe your doc will suggest a scan and then take it from there?
Let us know what you decide and the outcome as it is all helpful for others to read.
Are you feeling better generally on the HRT, apart from the bleeding?
Hurdity x