Just a general comment... This particular module/lesson always kicks my ass, and I've had to re-strengthen it many times. But it's usually because of "mistakes" that are just reasonable alternative translations that weren't accepted. Frustrating! But I try to remind myself that it's only software, and most of the time it's pretty darn good.
The only answers you're ever likely to get is along the lines of "you suggested _ as a possible answer. We now accept this answer." I don't think you'll ever get some kind of personal feedback from the mods unless you luck out on one of these discussion threads. But the people here are usually very helpful :)
It should be accepted, it is a fine translation.
I do want to point out something that I read in a comparable discussion on a different lesson: the construction with "у меня есть" is not usually used to indicate possession WHEN the item doing the possessing is not animate.
So, to say "I have a Х", we use "у меня есть". But, to say "The house has Х", we use "В доме Х". English uses "The has X" in both constructions.
For this reason, since the fourth grade is an inanimate set, the "в четвёртом классе" phrase is more likely to mean "the fourth grade has...", than "in the fourth grade there is..."