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I'm joining 4 rough-cut boards together to make a work table. There's about an inch of variation in width on each side of the board, so I have to plane the sides before gluing them together.
How can I get a base "straight" line to plane against when both sides of the board are crooked? Ideally the 4 boards would plane down to straight sides, so the resulting joined surface also has straight sides.
What you are wanting to do would be better on a tablesaw because of the amount of material to remove. A Jointer or Planer is good for making very strait edges and tight joints, but with 1/2" of variation, you're not to that point yet.
What you want to do is a form of a "tapering cut" on a table saw, but instead of tapering, you just want to make the edge straight. Tapering is when you have a board that is 6' long, and the top needs to be 3" wide and the bottom needs to be 4" to make up for a crooked wall or similar.
http://www.finehomebuilding.com/2014/09/25/how-to-cut-a-tapered-board-on-a-tablesaw
There are many videos and how to's if you search for making a tapered cut, and they usually involve attaching your board to a straight board at an angle. You'll do the same thing, but without the angle.
I've done this with boards as long as 8' and a taper of 1/2" or so relatively easily.
You can use a straight-edge guide on your table saw, as suggested by @JPhi1618 in the comments, or in this answer from the woodworking stack.
You can use a track saw (retail or home-built), or a circular saw with a straight edge clamped down.
A jointer will work, but will take quite a few passes to remove a 1/2" variation.
You can go old school with hand planes (the longer the better), but removing the bulk of the variation may be easiest with a (rip)saw, handheld or powered.
