This would be three days after a missed period so not a lot would be visible.
ANSWER
A yolk sac would definitely be visible. Even though you are 4 weeks and 3 days pregnant your baby is only 2 weeks & 3 days old, so you won't be seeing much of a "baby" for a few more weeks.
New Answer-
At 4 weeks, nothing would really be visible Not even a yolk sac. It usually doesn't appear until week 5-6. I am 4 weeks pregnant right now and nothing has really shown up yet.
You can tell you are pregnant through ultrasound as early as 4 weeks with a vaginal ultrasound.
5 weeks is the earliest most women get theirs (3 weeks after conception). They will have to do a vaginal ultrasound to be able to see anything and it won't look like a baby yet.
Fetal heartbeat
Generally, the answer is NO. However, during weeks 3 and 4 when the gestational age (or fetal age) is 2 weeks, the earliest change that can be seen with a vaginal ultrasound is the "decidual reaction" which is the thickening of the endometrium. The endometrium lining thickens as the blastocyst burrows into it. This cannot always be detected by ultrasound-sometimes it may take a special eye or hi-resolution equipment to see this "reaction" in the endometrium lining at that very early stage. And remember that when using ultrasounds, it is the transvaginal ultrasound which can detect development in the uterus about a week earlier than a transabdominal ultrasound.
Using transvaginal ultrasound, a fetus is detectable by ultrasound at around 5 weeks. Even a heartbeat can be detected.
About 5 1/2 to 6 weeks you should be able to see development on a vaginal ultrasound.
Around 7 weeks I saw the fetal pole at 5.4 weeks
You really need to talk to your doctor or the person who did the ultrasound about this.
As soon as the embryo or fetus can be seen on ultrasound. Around 5-10 weeks a embryo, or fetus at 10 weeks, can be detected on an ultrasound and can show if the baby is in utero or if it's eptopic.
I understand what you are saying when you say messuring, they messure the sac and it calculates what the size is to get the age of the sac/fetus.
Generally a heartbeat will be detected by internal ultrasound by 61/2 to 7 weeks but the baby is so small it may take a week or two longer. Be patient
Unlikely to hear but should be seen. Give the little one another 4-6 weeks.