The whole Extended/Priority service thing began some time after BI 4's release.
Though it says you need a support plan in order to install minor updates, this has never been enforced. Anyone can install minor updates. I hope BI never tries to enforce such a rule, because it would be a big middle finger to everyone who gets stuck on a buggy BI version when their license expires.
Anyway, Version 5 is due out June 1st. One way to buy BI 5 is to buy a year of extended service before then. I imagine you could also buy the extended service after then for the same effect, but the developer has suggested that pricing may increase when BI 5 is released. I imagine you could also skip buying extended service altogether, and just buy a BI 5 license (possibly at a discounted rate) after its release, but what this would cost is unknown.