You don't want a 4 foot wide worktable. it is too wide to reach over. 2 feet wide is just fine. You'll need a wider area for scarphing 4 foot wide plywood, but you can use two 5-foot long pieces of 1x6 placed perpendicular to your tabletop to hold the end areas of the scarf joint. Covered with saran wrap so they don't permanently adhere to the joint, they will provide support and give you a surface to put clamps on.

Go with 2 eight-foot long tables. You can join them with bolts to make your 16-foot-long table, and then disasemble them for storage.

Have the lumber yard cut a 4x8 panel of plywood in half for you. Two reasons: First, they'll usually do a free cut for you if you ask. Their panel-saw setup will do the job faster than you can start to set up a straightedge -- and if they screw up the cut ( sure, they are pros, but these things happen) just tell then that you NEED 2 good panels, and they'll go get another panel, rip that, and give you two good pieces. On the other hand, if YOU mess up the cut you'll pay for the experience. Second: the narrower pieces are half the weight and easier to handle as you try to get them out of the store and take them home.

Don't use sawhorses to support these tables. Use legs cut from 2x4's which are simply screwed or bolted (not nailed!) to the 2x4 frame under each table.
There are a few reasons for this:
The sawhorses assembled with the cheap metal brackets, tend to wobble.
Each sawhorse needs 4 legs, and you need 2 horses for each table. With 4 horses supporting two tables that means 16 total legs. Each horse needs a crossbar, too, so that is another 4 pieces of wood to buy and to cut. Without all this cutting and assembly, you just need 4 legs to each table
It takes one extremely patient person to move a table supported by sawhorses -- slide the horse a bit, shift the table, slide the other horse, repeat the process. Really, the job is easier when done with two people, with each one sliding a sawhorse-- and that is just for the 8 foot table. The 16 footer is more of a challenge. But if you eliminate the horses and just have legs on the tables then one person can move an 8-foot or a 16-foot table with ease.
If the legs are nailed on then they are difficult to remove, but if they are attached with 2-1/2 inch or 3 inch deck or drywall screws, or with 3-1/2 inch long, 1/4-inch carriage bolts, then these threaded fasteners can be easily removed and the legs drop off.
The sawhorses must be wider than the table. Otherwise, a slight bump will knock the table off of the sawhorses. Being wider, though, you tend to walk into the horses and bump things. It is a vicious circle. Legs, though, are mounted underneath things, and relatively out of the way.

You can use 1/2 inch or thicker material for the top. The main choices would be plywood or OSB. You support that on a "ladder" made of 2x4s. For each 8 foot table you need two 8-foot long 2x4s for each of the long sides. Between each of these place pieces which are 21 inches long every 16 inches. You'll need 7 of these for each table. If you cut 3 of these from an 8-foot 2x4, your remaining piece will be just under 33 inches, which is a fine length for a table leg. (You'll want to make sure the legs are all the same length, so eventually you'll trim them all to 32 inches long.) So far you've used 4 8 foot 2x4's, and you need two more legs and 1 more "ladder rung" Cut those pieces from a 5th 2x4, cutting the legs a bit long. For two tables you'll need 10 2x4's for these parts, and one more for bracing -- but you might as well buy a dozen.

To assemble the "ladders" take each of your four 8-foot 2x4s and mark a center line 48 inches from the ends. Measure 16 inches form that line in each direction and mark another line. Then make lines 32 inches from the center line in each direction. If you measure just one board you can use that as a gauge by aligning it with the others and extending the lines.
On each of these lines start 2 16-penny nails. Set two of the 21 inch "ladder rungs" on end and set one of the 8 foot boats on top of these. Align the "rungs" so they are each centered under a line and nails, and pound the nails in. After you do two "rungs" the rest go easier. The ones at the ends you don't need lines for. Just get them flush with the ends of the 8-footers. You may want to drill holes for your nails at the ends so you don't split the wood. Or, hold those on wih deck screws. After you do one long piece, and it looks like a large comb, flip it over and attach the other side.
After assembling all the "ladders", squaring them and nailing or screwing the tops on, you should trim all 8 legs to exactly the same length. Mount the legs to the inside of the long side rails. You cana put them at the ends, but I prefer to have them a little over a foot in from the ends -- closer to that first crosspiece. Again. use deck screws or bolts.
Once the legs are on, connect them with a diagonal bracing piece which can be cut from your left-over 2x4, or made from other scrap wood.

Hope this helps

PGJ