When it comes to being an FO, it all depends on where you get stationed and what job you may have. If you are in a line platoon serving with light infantry, you'll be spending a lot of time with the infantry. Basically your job is to be next to the Lt and be ready to call a fire mission. Most of the time you'll need to ask the Lt if he wants indirect fire. Infantry tend to forget that the FO is there. After the Fire mission is sent up, you'll need to make sure that the infantry doesn't move forward until rounds are complete. Of course, this is all simulated. If you are not with the grunts you will sit at the OP and call live artillery rounds. You can spend a week or 2 on the OP just lobbing rounds into the impact area.
We are called the King of Battle, but we are also the dogs of garrision. As an FO in the rear, there is not much to do. Once your equipment is clean, you'll be doing a bunch of class room stuff and details. Somedays You may find yourself getting paid to play your playstation all day. Other days you'll be task to do something that has nothing to do with being an FO.
Now if you don't want to be down on the line, volunteer to be at the BN or BDE level fire support. I have been there and hated life. My calling is down on the line. In the BN and BDE FSE (fire support elements), you'll be doing a lot of computer and radio work. Your job is to make sure no one is calling artillery on friendly troops and to "counter battery" enemy artillery. Also, you'll be around high ranking people. As an E-4, you will be one of the lowest ranking soldiers there. The best part about this job is the perks. You'll always have coffee, decent food, a tent to sleep in, and for the most part, you won't be directly in the elements.
I have not served with the tanks so I have no idea how things work there. I hope this helps out.