Mine are also 4 years apart, and we are dealing with a lot of the same issues with our son who just started kindergarten. He has lots of "green" days, but we've had a few yellows and two reds. He is acting out, not listening and doing what he's told, he has gotten into a fight with some other kids, and has been wetting his pants at school.
I've talked to his teacher, and both at home and school we are really laying the positive reinforcment on heavy. If he goes 15 minutes at school being good, she either gives him a sticker or some kind of verbal acknowledgment. He gets praised and rewarded (usually with M&Ms) when he gets home for green stars and not having an accident. If he has a good week, he gets a small toy. This has been our first solution to helping with his issues.
We are seeing an ENT this week - our son snores a lot, and we and our pediatrician think he might have sleep apnea and is suffering from chronic overtiredness. We're hopeful that this will help a lot.
And we try to give him as much one-on-one attention as we can at home, but like you know it's hard to do when there's a sibling that also needs attention. DH and I often tag-team - one of us reads/does homework with the older one while the other one plays with the younger one, then we switch.
Just wanted to let you know that you aren't alone. It's frustrating.