From Ptil Tekhelet:
This method defines a chulyah as between 7 and 13 twists. There are five knots, with chulyot between them of twists that alternate between Tekhelet and white. The first and last twist of each chulyah is white.
Comments by Rafi Hecht:
This is a method open to a lot of interpretation since Raavad holds 7-13 to refer to winds and not Chulyos. The second Chulya is practically non-important: it allows for all winds to be white for 7-8-11-13, it also allows for more than 13 winds in the event one needs to get a good 1/3rd-2/3rds ratio.
What’s moreso is that there’s a Machlokes Rishonim on starting with 2 and ending with three. Some hold it’s Krichos (winds) and some say it refers to Ksharim (knots). The Raavad seems to hold that it means by knots, which is why in the scenario of four Chulyos, the first Chulya is Meakev since there are two knots, one before one after, and the last two Chulyos are Meakev as you end up with three knots at the end. It’s also possible that this is how R’ Yehudai Gaon held like (courtesy R’ Yehoshua Yankelewitz).