It's an extremely fringe (and, IMO, wrong) theory that your instincts "cycle" with your tritype. You don't just randomly choose instincts; these people believe that your core has one stacking and that stacking loops.
Using my own as an example:
6w7 sp/sx/so - 1w9 sx/so/sp - 4w5 so/sp/sx
Using yours as an example:
4w5 so/sp/sx - 6w7 sp/sx/so - 9w1 sx/so/sp
As I said, I, and most people, think this is outright wrong - you have one stacking, period. That stacking applies to your whole type. Done. Instincts aren't just something you go through, it's something so embedded you struggle to shift your focus from.
Then again, I also think most people focus way too much on tritype, and this is probably a symptom of that, but whatever.
If you take for good the "classic" theory that enneagram fixations are subvariants of instinctual variants, then you would have your 3 cores of the same instinct. It makes sense. Plus, it's easier to type correctly since your core would take traits from other cores of the same instinct:
a SX 7 would draw traits from SX6, SX8, SX1 and SX 5.
Honestly I wouldn't take tritype into consideration since I consider it a really flawed and superficial theory, it's only a waste of time. Just my 2 cents.
There are a few tests online, and a few different websites explain the Instincts.
I don't really trust any of the websites that much and have ordered books about both. David Fauve and his wife have a book about the Instincts on their website, and some of the people they got the material from have written books of their own.