Yes, you are missing the boat in an important way, not that i don't admire the need for clarity high resolutions imply.
If this is a perspective with textures, a rendering of that size, were it possible in Archicad, would show evidence of pixel smearing and chromatic effects caused by standard, low-resolution Archicad textures. My advice is to do the best that you can with the 4048 pixels and add "Noise" to the image in Photoshop later to disguise what will be glaring flaws in the appearance of the surfaces.
It will render until Easter in any event, if'n it don't crash.
Secondly, it is an architect's conceit that resolution should always remain at maximum - ie 200 dpi - when the rendering gets big. You can easily satisfy your audience with a smaller rendering that is "upsampled" in Photoshop. Besides, once an image exceeds the normal eye/hand aspect ratio, people stand back. A rule of thumb is that you never need anything bigger than would fit on a letter sized sheet.
There is a swirling vortex of RAM demands once you get to the really large renderings - they also need really large texture maps to resolve properly - lots of RAM.
If you need bigger, get Artlantis. While it doesn't always make sense to try and make perspectives that huge, in Artlantis, it is feasible - Artlantis also makes gigantic orthographic views to large scale easily - where it pays off as 1:50 scale high rise building elevations!! Now that Artlantis is closely associated to Archicad files for simplified updates, and that it has a good selection of quick-rendering entourage, you will be way ahead investing in it.
Suddenly, saving the Archicad 3D output file is quicker because the polygon intensive entourage only lives in Artlantis.....
As to speed: we frequently render a dozen perspectives, building sections and elevations for development permits on 30x42" boards in Artlantis overnight on a single 8 core machine. This, usually being the night before the deadline, means a lot rides on not having a power failure.
Dwight Atkinson