Does every deck stair with stringers longer than 6 feet need mid-span support?
It is not possible to use a solid stringer mid-tread, so where the code addresses increased span allowance for use of solid stringers, there is a presumption that the middle stringers (if any) are cut. Your 48&quot; width will need two cut stringers, because anything exceeding 36&quot; width need max stringer spacing of 18&quot;OC. There is no support or span requirement that is dependent on stringer depth. Max span 13' 3&quot; (or 16' 6&quot; for southern pine). All stringers shall be minimum 2 x 12.
Your 4' wide 13' span stair will have plenty of support with the 4 required stringers...