Do you mean the child who is "away with the fairies" actually does have learning difficulties? If so, she should be getting help provided by the school. If not, she just needs positive vibes around her and help to focus. Remember children have learning spurts as well as growth spurts. I know what it's like bringing up 4 offspring - it's tough. You have to be the one to give the positivity, not easy when you're juggling so many balls in the air. Ask boy 1 if he really is unhappy about something and if you get a sullen "no!" Back, he's normal. Same question to boy 2. If he really is worried, see what you can do. If not, same answer as before. The girl who's not focusing, keep an eye on her but step up the positivity around her. The younger boy, just listen and if you hear him coming out with nasty remarks quietly tell him that is not acceptable and ask him why he's saying such things.
Last bit of advice: if these suggestions don't sit well with you, feel free to ignore them. They're your kids, you know them. Do what your heart tells you.