Buying Softwood Lumber for Shop Furniture

If you can process the 2x12 southern yellow pine from home depot, you can get really good wood from those boats and they are usually pretty dry. The board's I used for my bench didn't move at all.

BigCountry79, that's a beautiful quartersawn surface on your workbench.
csmcswain, take note that the 2 workbenches with solid tops are much narrower than the 4' wide work table you are thinking of. Narrow benches often start with a massive top, but wider tables start with a stiff understructure stable in both directions, not just one. I read your description and it sounds like a piece of plywood casework to me. You could build it with a framed structure if you are thinking it will be practice with solid wood joinery. For rigidity, this requires quite a few closely fitted mortise & tenon or bridle joints, particularly of you are going to put it on wheels. If you want to go that route, chose harder wood like Southern Yellow Pine or Douglas fir rather than the spruce/fir mix often referred to a whitewood and make some practice joints first. These are often described in terms of strength, but stiffness is frequently what is important.
I learned a couple lessons with this bench made entirely of Douglas fir LVL.



The structure is two large frames with slot and tenon joints at the corners. These are very ridged. These are joined by about 12 cross members using dowels and large screws. It swayed back and forth like crazy until I added the closely fitted "X" braces at each end

John's explanation is great.
But, most folks do not understand that all of the components need to work together for rigidity, for mass, for bending, and for resistance to racking especially.
One giant hunk of wood would be ideal, but not at all practical.
We want the top massive and stiff to absorb blows from hammering and striking.
We want the bench to resist pushing and horizontal forces from hand planing.
We want the bench to resist racking like an open carboard box will do.
If we fold all the flaps inward, it has no resistance to folding over and collapsing.
If we open all the flaps and tape them down, even a light weight cardboard box becomes rigid.
"X" braces work because a triangle can NOT collapse.
Triangles are used as braces in bridges for the same reason.
A sheet of even the thinnest plywood on the back and ends will do much the same, but will NOT add any mass.
Book cases need a back because books are heavy and when set in motion, the greater mass will keep it in motion.
An "open" book case relies entirely on the strength of the dados holding up the shelves, not all that much.
The mortise and tenon joints have good resistance to racking because the shoulders are tight against the legs to prevent any movement.

work benches VS work tables, VS cabinets which one?
As much as I admire real Roubo style work benches, I don't have one. I may not ever make one because my working style doesn't require that massive a bench.
Rather I have come up with a design that utilizes legal size two drawer file cabinets as supports for various types of tops:

I use slab doors with a 1/4" hardboard protective surface:


A torsion box made from 1 X 3's and 3/4" particle board:

A laminated Maple top with bench vises at each end;


Three file cabinets all attached, with a over hanging 2 drawer Formica top:




By using the file cabinets, bought used at a office supply warehouse for $75 each, I get heavy duty drawers and great structural support.
By attaching them together at the sides, they become "modular" and when used through the shop are interchangeable if necessary.
Torsion box top and the Formica top above, do not allow for mounting a vise in the 3/4" thick surface like the laminated Maple top.
The pair of Craftsman woodworkers vises at each end can work together to support a longer piece when hand planing an edge.

Each different style or "work bench" has a different use in my case:
Supporting the outfeed from the table saw combined with an assembly surface.
The slabs are used for bench top tools like drill presses and general utility tools, not as a "working" surface.
The two drawer Formica top is a light duty work surface on locking casters.