Before you go any further, look at this block. 
Do you see it is made up of 4 units, opposing corners the same? You have already experienced making 4 patch units in Block 2. You will do them the same way for this Block. The other two corners are called half square triangle. There are so many ways these can be made, but because we only need two for this block, I won’t go into all of them at this time.
Pull out your bag of 2 1/2″ squares and get out 4 dark and 4 light. I will leave you on your own to do the 4 patch units using the 2 1/2″ squares. Refer back to Block 2 if you need a refresher.
Next bag is 4 7/8″ squares; you will need 1 med dark and 1 med light. Put squares RST with the med light on top. Place your ruler so it lays on the diagonal of the squares and with your choice of fabric marker (I use a regular mechanical pencil), mark along the ruler from corner to corner. You will be stitching a scant 1/4″ from this line, so I suggest you place the ruler 1/4″ mark on the line and draw lines 1/4″ on each side of the diagonal. To allow room for the thread, stitch one thread width from the sew line toward the center diagonal line. What you will see when you are finished is your stitching line snuggled up next to the sew line you drew, but not on it. See the photo for clarification as needed.
Cut on the first line you drew from corner to corner. With the dark fabric triangles wrong side (WS) up, for just a moment hold the iron on the stitching line to “set” your thread. What that really means is the threads will relax with the heat and become more embedded in the fabric. Lift the corner of the darker fabric and lightly press the seam toward the dark fabric. You do not want to iron this as the seam is on the bias (the stretchiest direction of the fabric) and can be distorted leaving your half square triangle block “wonky”, what quilters call any piece that is, well, wonky – like stretched out of shape! Pretty clever way to get two blocks, don’t you think? Measure and if necessary line up the 45 degree angle on your ruler and do some trimming if the half square triangles are more than 4 1/2″ square. Those little “ears” sticking out can be trimmed off, as well. 
Now it is time to sew your units into rows and rows into Block 4. You have learned how to make half square triangles (HST). HSTs are fun to play with when you have several. So many designs are nothing more than HSTs.
I hope your eye is starting to see blocks in quilts and can see the units that make the block. When you can do this, you will know even the most complicated can be simplified and sewn one stitch at a time! 
If these blocks look a bit wonky, it is because I had not pressed them with an iron yet.
Until next week, keep working on that scant 1/4″ seam. You’ll get down by the time this quilt is finished!
Be sure and contact me if you have any questions or problems!
AND WHERE ARE YOUR EARS?
I started saving my “ears” a few years ago for fun and to maybe come up with some clever idea on how to use them. I’m amazed how quickly this quart size canning jar got so full! 