The System 4 came in a double height card frame and photo's (Chris's Acorns) show that it was simply two 3U Card Frames mounted on top of each other. The bottom Frame held a 14-slot backplane, the top Frame two full size 5¼" Floppy Disk Modules.
The Card Frame is 6U high, 84HP wide and 180mm deep (1U = 1¾" or 44.45mm, 1HP = 5.08mm).
The left hand Front Panels are 14HP wide, the lower position would hold the PSU, the two Floppy Disk Modules are 35E wide, the lower Card Frame has room for 14 5E wide Front Panels.