If you can't get a 4-patch to be exact, you will never be able to make 9-patchs, can't trim those, or anything with triangles. work on the accuracy of the 4-patch until you get it right. I test my 1/4 in seam allowance by cutting a piece of 1/4 grid graph paper so I have a good edge to put against my seam guide or the 1/4 in foot guide. lower the needle until it nears piercing the paper. I adjust my needle until I can get it to go through just to the right of the 1/4 in line. Then sew down the graph paper for three or four inches to check that you are staying online. Now sew a four path together and if it is accurate mark the line on the dial used to change to needle position. If that didn't work, move the needle a tiny bit more and test again.