Another AZ girl here. MSQC has a fun way to do 4 patches from strips.

Your extra 1/8” is due to you cutting your squares 1/8” larger, ( 3 7/8” vs 4”) and you sewed perfectly. Great job. Now, you can trim your squares to your 3 1/2”. Or you can cut your squares 3 7/8” and ( hope) to continue getting them all perfect.
I would continue with the bit of trim room.

The Eleanor Burns 1/2 square ruler makes this so easy, you use the oversize pieces, (I cut mine 1 inch bigger than the finished component) draw the diagonal, sew 1/4" on either side of the diagonal. Then, BEFORE you open those pieces, you cut it down with the ruler. It's perfect every time with no worries and it's quick. Wasted fabric is minimal.

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i agree..EB methods/tools are so to the point....she is so willing to share her knowledge and her rulers are reasonably priced and if used correctly perfect results! Love her recent videos on YouTube

I cut my squares about 1/2 inch larger and sew 1/4 inch on both sides of the diagonal line just like you did but the problem may be with ironing. I find that I can't use steam and I can't push the iron across the square. It sounds like you could just trim your square to 3 1/2 and it will be fine. Good Luck

how to sew a 4 patch block two at a time
 

This is sew easy you have to try it. Very well explained!!
https://www.youtube.com/watch?v=alReWeIynGw

Thank you for this and I will try it.

Personally, I like Deb Tucker's methods and tools for making PERFECT HSTs................There are no bias edges to get stretched, and every block winds up being exactly the size you need. Check out her video for the Tucker Trimmer on her website............very informative...........Deb Tuckers Studio 180 Design.net