6 instar or 7 instar?

i know this ain't the portion where you post and ask what instar is your scorp.  i cant find it here. anyways.
here's the story:

 i have two h.longi, 1 6i, and 1 4i. my first longi i got was said to be 3i. and i was assuming it turned 6i on aug. 6, 2013. and my 2nd longi turned 4i today. but as i was looking closer on my 4i longi. i noticed it's about the same size of the  3i exoskeleton of my first longi.



then it puzzled my mind if my first longi i got was already 4i instead of 3i and turned 7i on aug 6, or the seller was correct that it was still 3i and turned 6i on august 6.

here's the pic and measurement (inches) of my 6i/7i longi and my 4i longi:



thanks in advance! hope i could get the answer that i want. Godspeed Team ASA!