Many of us blithly throw away the other half of the square (triangle). But in frugal years gone by, that never would have happened. Let's see, if I understand you correctly you want to cut triangles, that when sewn together will form a 4" square, because when that is pieced-in it will become 3 1/2" .
A rule of thumb is to cut the side or leg of the triangle 3/8" longer than the size you want when it's finished (or in this case it could be a square giving you two triangles).
Therefore, if you want to have a raw 4" triangle you would cut the new length 4 3/8".
A. You can cut the square diagonally, place 2 different colored triangles together, and then sew a 1/4" seam along the hypotenuse.
B. Or you can 1.) Place two 4 3/8" squares together, 2.) draw a line, diagonally from corner to corner on the top or lightest one, 3.) stitch 1/4" from that line on both sides of it, and 4.) cut on the drawn line to have two finished half-square triangles.
The 3/8" rule applies no matter what size you're making.
Best of Luck!