Agree with Don P. 2x4s are for interior walls. A good rule of thumb for spans is take have of the width of the nominal lumber and and at it to the total width and that gives you your max span in feet. Example: 2x4 - 6 feet, 2x6 - 9 feet, 2x8 - 12 feet, etc. Not an exact span but good for a quick ballpark estimate. If you are building a shop there will be quite a bit of weight on it. I would want to start with a good foundation. Chalk-up what you have as expensive fire wood or salvage the 2x4s for a wall, assuming you didn't glue the subfloor. My 2 cents.