Hardwood thichness are sized by the 1/4 (or quarter) of an inch, based on the rough-cut size of the wood. 4/4 started out as 1", 5/4 - 1.25", 6/4 - 1.5", 8/4 - 2", 16/4 = 4".\
Why don't they just call it 1x, 2x, 4x like construction softwood? When you buy 16/4 you get wood that's typically 3-3/4 - 3-7/8" thick (vs 3-1/2 for a 4x4), depending upon the amount of planing they (or you) need to do to get it flat, smooth and even.
The big box stores typically sell it as 1x or 2x and plane it down to the mass-production thicknesses used for construction lumber and sell it by the lineal foot. In those stores you get less wood (albeit a consistent thickness) and pay far, far more for the same wood.
A couple of months ago I found myself needing a piece of red oak on a Sunday for a rush project (typically my only shop day), so went to HD. I ended up paying $48 to complete my project that day when the same wood would have run me $20 or less at my regular lumber place. Unfortunately they are only open 8-4 M-F so I have to pre-plan to get time off work to go there, where I usually "stock up".
This is the reason why so many of us own jointers and planers.. it doesn't take all that long to pay for one... or at least you spent the money buying a tool instead of giving it to HD. <g>
Check out that new high-tech cordless router.. wireless and no recharging required!!