I would counsel going with the 27...
Builds with screen much bigger than the tablet active area
(on all sides) tend to be remarkably jitter-free, as the T-Con board and the discontinuity added by the LCD borders can be kept out of the way.
http://forum.bongofish.co.uk/index.php?topic=2218.0 (this uses an Intuos 4)
http://forum.bongofish.co.uk/index.php?topic=2458.0 (this uses an Intuos 2)
On the other hand, as Ertew notes, the best cover for a 40" 4k is using the two digitizer placed verticals, with a "respect band" between their active areas
(even in the best case scenario, you can't go below the width of one "digitizer border" of distance between the two, by overlapping a pcb to the other, with in the middle a grounded metal strip to intercept the mutual interferences; in the worse case, the "side band" interferences are strong enough that not only no overlap of the PCBs is possible, but rather the boards must be kept completely separated).
In this configuration, the screen is only slightly taller than the boards active areas
( by half an inch), which means that the T-Con board would lay directly above some the digitizer antennas, who would pick up its em fields (and RF-Cage would be needed almost for sure) and -thus - add jitter.
The "dead area" in the middle is not avoidable, and means that one must chose one of the two sides as "drawing area", and leave the other for palettes and auxiliary windows.
Now, I use two builds in a similar way - a slightly jittery 21.5" as main drawing board, and a pretty precise 9.7" on its right side as palette screen.
The two screens are separated by about 6 cm of distance (2,5") - and it is already a bit of a bother.
More importantly, even if I sometimes desire to place them together in a single case, the truth is that I still rather have the small one able to be tilt in my direction, and so reduce perspective aberration and parallax when my head is above the drawing screen, rather than a more elegant, flusher
(but, actually, less functional) solution.
- Truth is, it already is a bit distant from my drawing station as it is.
In a 40" with digitizers as near as possible, the situation would be similar but there also would be no way to tilt half of the screen in the user's direction.
As for placing the two digitizers near the LCD borders, with only one big "dead area" in centre...
if 2" are already a bother, 10" would be unbearable.
In conclusion, while the 40" would look amazing, I am not sure that drawing on it would be all that fun.
Note: edited for slight orthography and language fixes.