Can I use a segmented abrasive blade in my circular saw for cutting drywall?
You ask if a 4-1/2" blade will fit a 4-3/8 saw probably not, but take the saw to the store and ask them to let you see if it will fit.
Using a saw to cut drywall? I usually use a razor knife, or a jab saw when cutting in old work boxes, I have never seen someone use a circular saw to cut drywall, I would hate to see the mess that leaves. Change over to a razor knife if you want less mess.