Thank you for raising this, this is very interesting to read and know as will impact me too.
I do wish to add though, that I’d you apply before doing 4 years, the IMechE have been known to “humble” you and effectively grill you until you trip up and suggest you reapply with more experience. Unless there’s a rush, might be best waiting for 4.25 years (especially if you didn’t go down MPDS route)