What size should a gestational sac be at 5 weeks 4 days? is 4 mm ok?

About 1 cm: Usually the sac will be larger. Usually you will start to see a yolk sac then. But this is the dilemma. If the pregnancy implanted 1-2 days later than you think it can alter what you see. So a repeat us 1 week later is much more accurate.

Pea size: It should be about the size of a pea. So 4mm should be ok.

Mean sac diameter: The msd is one measurement used to estimate gestational age. The crown rump length is more accurate once a fetal pole is visible. General rule of thumb with msd is to add 30 days to the # of mm's. A sac of 4 mm would, therefore, suggest age of 34 days or approximately 4 weeks 6 days. You may just be earlier than you think, which happens quite often.

? Dating Criteria: 4mm is more common for about 4'6 to 5' but could be normal at 5'4. It's certainly not far off. The ? Though is how were the dates defined and how accurate are they? Only time will tell. Know it's hard to wait. Best of luck.