They mention a 4xMRT limit, so 4 x 4x32bit follows on from that anyway, seeing as 4x32bit is the fattest texture format
But yes, a "feature level" API would be nice, so you can ask the device what kind of limits it actually has.
BTW, I wouldn't dare use 4 x 4x32bit on the nextgen consoles! That's way too much bandwidth -- At 1080p, that's over 120MiB of framebuffer!
(or if you assume a perfectly optimal only 1 write followed by 1 read per pixel, then at 60Hz that's 14.8GiB/s of bandwidth)
3 x 4x8bit is a pretty standard setup for deferred rendering, and even that's uncomfortable from a bandwidth point of view...
Keep in mind that on Xbone, you want to keep your frame-buffers under ~30MiB! 3 x 4x8bit plus a depth buffer just barely fits in 32MiB of ESRAM.
3 x 4x8bit isn't uncomfortable on next-gen consoles. I guess Xbone has more restrictions here, but Infamous Second Son used 6 1080p gbuffer targets + depth + stencil - 3 of the gbuffer targets are 4x8bit, the other 3 are 4x16bit. See slide 41 here: http://suckerpunch.playstation.com/images/stories/GDC14_infamous_second_son_engine_postmortem.pdf