My lmp was 14/5. Hpt came out positive with faint line on 18/6 and 20/6.went for my first ultrasound on 11/7 (supposed that 8 weeks around as per lmp)but u/s result states gestational age of 4 week 3 days.with gestational sac diameter as 13.4 mm nothing else.
I have history of irregular periods and one missed abortion
Please advise if the gestational age advised coresponding to the sac is correct

You should have been asking your OB/GYN these questions.

If you are concerned about when the pregnancy occurred because more than on man can bee the father. You will have to get a DNA test done as these other means of determining conception dates are only guesses and can be off as much as two weeks in either direction. Meaning a possible 4 week window when it occurred.

Any calculation of gestational age or conception dates is educated guesswork. This means it can be +/- 2 weeks or maybe more.

The only reason I see that this should be of major concern is if there is a question of paternity. Anyone who inserted their penis after May 14 is a potential father. The only way to be sure is with DNA testing after birth.

With just gestational sac measurement, and nothing else seen (yolk sac or fetal pole) it could put you roughly 4-5 weeks fertilizational, or fetal age. While your period may have been 8 weeks ago, it is possible you ovulated a bit later than the average, which would account for the difference and what was seen.

A scan that early doesn't tell you a great deal as what can be seen can change in just days. It can also depend on the experience of who is looking at the results.You will have a better idea how the pregnancy is progressing, and if there would be any concern, with another scan in just 2 to 3 weeks. You'll also learn if the estimated dating is consistent with what would be expected from the first scan. Do you have another scan scheduled?