OK... I should read the link before posting!
The Southern League will be running the new - and as yet untitled - Step Three "Midlands/Central League Premier Division" which will be at the same level as the Isthmian Premier, Southern Premier and NPL Premier from 2018/19... which makes sense.
However, the Isthmian League will be running the one new division at Step Four - which will run alongside Isth 1N, Isth 1S, SL Central, SLS&SW, NPL 1S and NPL 1N.
That's somewhat odd, but it's only odd as everyone assumed the new Step Four division would be a direct feeder for the new "Midlands/Central League Premier Division" - in much the same way that there was only one NPL Div.1 feeding into the NPL Premier until fairly recently.
Everyone has also assumed the new "Midlands/Central League Premier Division" having only one feeder division was a temporary measure for a couple of years, giving more clubs the time to satisfy ground-grading requirements and apply for membership of a second regional "Midlands/Central Premier League Division One."
Clearly there would've been "Waaaah! Unfair!" squeals if that assumption had been correct, as the present "Champions promoted; 2nd, 3rd, 4th & 5th to play-offs" would have continued to apply in the six regional Isthmian, Southern and NPL divisions, while "Top three promoted; 4th, 5th, 6th & 7th to play-offs" would presumably have applied in the new "Midlands/Central League Division One" - either that or the new "Midlands/Central League Premier Division" would have run with only two clubs being relegated, which would still have led to "Waaaah! Unfair!" squeals.
It's still somewhat clumsy that the Southern League will oversee the Westcountry and the Midlands up as far north as Cheshire, Derbys, Notts & Lincs and maybe East Anglia as well at Step Three, while the Isthmian League will seemingly be overseeing Cambs, N'hants, Leics, Oxon and maybe bits of Warks at Step Four.
Going straight to 1-2-4-8 would've made more sense in some ways, but ground grading rules and other problems for clubs moving up a level mean a few transitional seasons of 1-2-4-7 is more practical.