The 7/8 measurement assumes that you will sew a perfect seam, so if you cut your squares at 3 7/8 you should not have to trim anything. The pattern says: "
Trim to measure 3-1/2" if needed..." Cutting your starting squares at 4" will give you a bit of wiggle room, because often you will sew the seam too large and your square ends up smaller than 3 1/2. You can't trim it & make it bigger
. Most folks find it preferable to cut the starting squares at 4", then trim just a tiny bit. If you haven't cut your fabric yet, I would cut a pair at 3 7/8" & a pair at 4", make your HST and see which one you prefer.