Answer
We look at the 3rd and 4th carbon to draw the most stable conformation because it gives smaller substituent groups such as methyl and ethyls. If we look at 1st and 2nd or 2nd and 3rd carbons, substituent groups will be butyl or pentyl, which are bulky and unstable.
Work Step by Step
When looking at the 3rd and 4th carbons, the largest substituent groups are ethyls. We want the ethyls to be anti from one another to give the most stable conformation. See image.
