If I am Just going to touch up the edge on a knife or blade can I just use the fine grindstone on the T4 or should I always start with the course?
I don't know what you mean by "touch up" but if is just barely dull, use the leather honing wheel first after applying more compound. If that does not do the job for you, and it is only in need of touching up; use the fine grade of stone. Use the grader to make the stone fine.
I use the coarse grading of the stone only when a knife has damage or needs some serious restoration. On a really dull knife I use the stone somewhere in between coarse and fine. This may require a couple of extra passes at the midpoint of the stones grading. This is where a coarsely graded stone ends up after a number of knives. I then use the grading stone on the fine side and finish up with the stone as fine as I can get it easily. I have honed my knives that are just beyond where a steel will bring them back but most customers bring them when they are much duller.