Finally! A question I feel qualified to answer!
So I've used the 4000 for about a year now. Initially it was fine, and relieved the pain I felt from using the Apple aluminium knife keyboard. For the past couple of months I've been craving something better. I have large enough hands yet I've gotten pain from stretching to hit keys on the 4000.
I then discovered the ergodox, which is the first mechanical I've owned since the model m some 20 years or so ago. Today was the first day I used it full time. Believe me it felt good to unplug that 4000 and stuff it in the cupboard.
I should point out now that speed doesn't matter to me. I'm a full time programmer so I break to think more than I spend typing.
I don't think there's anything wrong with the design of the 4000. I particularly liked the padded wrist rests. But those keys.... After you try some mechs you realise why people hate them so much. They are bad, even for rubber domes.
A full size mech will suit you if that's the kind of keyboard you want. Just choose your switches carefully. I use browns in my dox, and I love them. Easier to press than the 4000, yet just tactile enough for me to not bottom out.
As for accuracy, after just one day I'm more accurate with the dox. This could be because my WPM has dropped, but I believe it's because it's just a better keyboard.
I did try the kinesis advantage but just couldn't get on with it. For me it was far to high to use comfortably.