That is very impressive knowledge, berlinonuat and fietsbob.

Where is the extra 1" located or how was it achieved?

I had a Mk 3 frame apparently. Is the Mk4 a worthwhile upgrade from Mk3 for current and former owners? In other words, does that extra inch makes a noticeable difference in performance such that one should consider another purchase?

Adding to answers so far for the OP, the Mk3 Brompton was acceptable in my opinion for a bike of that nature but not really all that great of a ride experience either. I used it in the city and along parkland trails. In the city it's excellent for convenience, along hardpack and light gravel trails, it seems okay. Not being an expert, I can't offer much more than that general subjective description in terms of ride performance. Of course, you buy a B for the fold foremost.