Would an ultrasound be able to tell the difference between a fetus at 4wks3days and 7wks3days from conception your scan showed a faint flicker of a heartbeat which date is it more likely to be?

Absolutely a scan can tell the difference within 3 weeks at this stage. The heartbeat would be visible at both 7 and 10 weeks (which you would be pregnancy wise). Measurements are taken to date the pregnancy so you need to ask the doctor/tech/nurse how many weeks pregnant you are.