(1) When you talk about *what* a systems administrator does, it doesn't sound that hard: installing and configuring software, patching, installing and configuring hardware, researching and comparing potential upgrade options, troubleshooting problems, etc.
What 4th graders probably don't think about is that none of these things by themselves may seem particularly hard at the scale of an individual computer, but when you multiply each of these activities by a gazillion servers, routers, clients, etc., then it has the potential to become a real nightmare. So you have to use tricks & technologies in a company's computing environment that you'd never bother with at home.
E.g., "Ever seen your mom or dad install a Windows update? Remember how nuts that made them? Now imagine doing that across 20,000 desktops in 10 cities, and being given only 3 days to get them all done!"
(2) Probably a lot of your time is spent being a detective, trying to puzzle out why something that oughta be working ain't. Telling stories about some of your successful detective adventures might be entertaining.
All people (including kids) like to be told stories, so the more you can populate your presentation with interesting anecdotes, the better.
And, as one person already wrote, bringing some old or broken hard drive, circuit boards, etc. to pass around the classroom probably couldn't hurt either.
Also, many 4th graders I know think that the *monitor* is the computer. They point at it and say, "That's the computer, isn't it? Why are you fiddling with that other box?" I know that sounds crazy, but that's the way many 9 year olds think. So don't assume any understanding of computers just because they know how to play Spore.