Let's make 4-at-a-time half square triangles!
Step 1: Starch Your Fabric
One major thing to know is that with four at a time, you will end up having to cut diagonally across the fabric, which will give you biased edges. It's really important that you starch your fabrics prior to doing the four-at-a-time method so that you don't end up stretching the fabric as you are sewing it.
Step 2: Place Two Fabric Squares Together
Take two squares that are the same size and place one square on top of the other. I usually put the lighter piece on top because I can see the seams a little better. Make sure they are right sides together.
Step 3: Sew 1/4" Around Squares
Sew 1/4" around the edges of the squares. You can easily turn your squares as you get to each edge in order to go around the whole square.
Step 4: Cut On the Diagonal
Cut an X going from corner to corner. You will get four pieces that will be your half square triangles when pressed open. I usually press towards the dark side. Be careful when pressing so you do not distort the edges.
Step 5: Trim
Trim your half-square triangles to the size that you need, cutting off all dog ears in the process. Again, be careful when trimming so you do not distort the edges of your fabric.
So there you have it, super easy 4-at-a-time half-square triangles.
Need more help? Here is a short video showing how they are made: