Well, CD4 was just an extension of CD3 as far as I know.....by that I mean not all new, whereas I think CD6 will be closer to all new. CD4 may have been limited for utility use larger than Edge/MKX and may not have been ideal for Explorer/Aviator use (that's just a guess on my part). They were probably also trying to finally get some life out of D4, and Explorer had been doing well, so figured they could divert resources to other projects with D4 living on for a while longer, and at a certain point, working on a CD4 version to only replace it shortly thereafter with CD6 just may not have made sense.
Just to clarify,
CD4 (evolution from EUCD) replaced CD3 and CD3S which were mazda based platforms.
Originally Flex was supposed to be on CD3S (mazda) but was changed to D3, hence the
paradox of CD3S Edge/MKX and and D3 Flex MKT at the same plant Oakville)..
I too have been wondering why Ford had not proceeded with a CD4 Explorer/Aviator,
It could be that the CD4 platform has reached the upper limit of its design envelope and
changing platforms would not give Ford any real or immediate advantage over current products.
I know that sounds like a cop out but it does kinda explain why Ford hasn't proceeded with
a replacement for Explorer / Taurus, they may be more than pleased with D3's sales results,
CAP is unching out upwards of 35,000 units a month, so why change.