To keep the list concise, I'm assuming both parents have the same number of perfect Roman numeral 4s (and the perfects are in the same stats) and not looking at every possible combination.
The perfect Roman numeral 4 has a 1/2 chance of passing down. 3 are guaranteed to be random, and the remaining 2 or 3 are guaranteed to be imperfect. Average: 19/32 perfects
With destiny knot: You're guaranteed to get one random Roman numeral 4 with a 1/32 chance of being perfect. There's a 1/6 chance that the random happens to be the parent's perfect one, so you get 1 perfect the other 5/6 of the time. Average: 83/96 perfects
So if your parents already have perfect Roman numeral 4s, and you can use nothing or a destiny knot, then you should start using destiny knot immediately. However, this assumes you can't use any other items, such as power items.
With power item: You're guaranteed 1 perfect, 2 imperfects, and 3 randoms. Average: 35/32 perfects
With destiny knot: There's a 1/3 chance that the random Roman numeral 4 is one of the parent's perfects, so only the other perfect one gets passed down. The other 2/3 of the time, you get 2 perfects and 1 random. Average: 163/96 perfects
With power item: You're still guaranteed 1 perfect and 3 randoms. There's a 2/5 chance that the other perfect Roman numeral 4 passes down. Average: 239/160 perfects
So use nothing for parents with 0 perfect Roman numeral 4s, a power item for 1 perfect, and a destiny knot for any more perfects. (though definitely consider the fact that this answer has at least a 1/4 chance of being wrong)