Cut the pine boards to a length several inches longer than the planned final length of your beam. If you have a choice, select the straightest and flattest boards that you have available.
Lay the first board on a flat surface, and cover its upper face with a zig-zag line of construction adhesive. Lay the second board on top of it so that their edges are even. Drive several 2-inch-long screws through the two boards to hold them together.
Spread construction adhesive on top of the second board, and lay a third board on top of it. Tack this third board onto the other two with several 2-inch-long screws.
Spread construction adhesive on top of the third board, and lay a fourth board on top of it. Drive 4-inch-long screws through all four boards to secure them permanently together.
Cut the assembly to the desired length with a circular saw.