That depends on a number of factors, including diet, warmth and even the grow rate particular to the turtle. With many captive turtles, given the high protein diet they tend to receive, I think the four inches is reached at about a year (some keepers turtles have even been larger than that at that age). This is not, necessarily good, however--you want your turtle to grow slowly, but steadily, and if your turtle doesn't have a shell length of 4 inches at a year (but isn't really small), if s/he's healthy otherwise, that's good.
Some turtles will show secondary sex characteristics at less then 4 inches. My RES was less than 4 inches (around 3.5), but with his long nails and shape of the tail, I was very sure he was a boy.

"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-