make them both flat....?

You can make them both flat if you have a flat reference surface OR can use winding sticks and a straight edge. A reference surface will show you immediately where the twist is by pressing down and see how it rocks. Then you take the two high points down until it stops rocking. Repeat on the second piece until they don't rock and then see how they mate together. That's what I would do IF my 6" jointer blew up....

:|
When you get them close, you can use several pieces of sandpaper and sticky tape on the reference surface and fine tune them.

 

What about using a western cedar or pressure treated 4x4? There are even places that have whitewood 4x4's but they are few and far between.

Making a 4x4 out of 2x4's would maybe only yield 3"x3" even if they were flat to begin with. The boards are now 1 1/2"x 3 1/2" with rounded over corners so you would need to trim 1/8" to 1/4" off each edge. You could maybe make 3 1/2"x 3 3/4" if you used 2x6's and a 1x6. It would give you some material to rip off the rounded corners.

It's just going to be a lot of work to hand plane lumber to glue up. They had us doing that in High School and as soon as I graduated I put up the plane and purchased a jointer. Still a lot of people do that out of choice. It may be your plane isn't sharp enough. In order for the plane to function properly it needs to be razor sharp. You just place the boards together and see where the high and low places are and take light shaves and keep putting the boards together to insure you are not taking too much wood off. It's just one of those things that require a lot of practice.

 

Never seen a woodshop bench that needed 4x4 legs. Design/effort/time/materials/money better spent on rigidity.

 

Any suggestions?

Buy a 4 x 4.....[emoji848]

 

Thanks everyone, you are probably right. I guess I will pony up the extra money and get some 4x4s, lol.
On the bright side, after about 6 hours of work I managed to get 3 of them flat. I suppose it was good practice for my actual purpose of the bench, which is to build guitars.
The boards only ended up being about 1 - 1/4 after flattening one side, so they would have been 3 - 1/2 x 2 - 1/2 after glueing.

 

There are workbenches and there are workbenches, so I guess it depends on the kind you are building, if this is a utility grade project just glue the stock 2x4's together and trim them to 3x3 for the legs. If it is a show piece then that is another matter, but the bench will have no bearing on the quality of your guitars.