I'm a bit confused about how renewals work, since 10 years and 4 years don't overlap cleanly.

Specifically, my second passport expires this year, while my primary passport expires next year.. Am I understanding correctly that when I renew my second passport this summer, it will get a new expiration date 4 years later (even though for 3 of those 4 years, my primary passport would have been expired), and then next year when I renew my primary passport, I'll get a new expiration date 10 years later?