Thin lining

I had a chemical last month after ttc 9 months. When I was having the miscarriage, they did an ultrasound that showed my endometrial lining was 4mm. I looked it up and read that that's terrible for implantation.

I had a followup today two days before my period is due and lining measured 4.5mm. The ob/gyn said that I shouldn't worry because there's no cutoff where you can't get pregnant. But what I have been reading suggests that 4.5 is inadequate.

Any experience or information that might be helpful?