Low HCG levels at 4 weeks 6 days?

That's not low! I had my hcg checked at 3 weeks 5 days, and it was 13. At 4 weeks it was 39, I retested at exactly 5 weeks and it was 936! My dr thought 936 was great for 5 weeks and yours is 900 earlier than mine was. I would guess your hcg to be around 3600 with your next check. This early in pregnancy it usually doubles every 48 hours at least. I had an ultrasound at 7 weeks and we saw the sac, the yolk, the baby, and the heartbeat :) You sound like you're fine! Good luck!