My 4 Yo Is a Moody, Cranky and Mean

Is he tired or hungry at these times?

is he vying for your attention esp w/the baby around now?

if he no longer naps, how about just some relaxing, "down" time for both of you. Lay down on the sofa or bed for 30 mins. Lay on the couch & watch something on tv on very low (some kind of relaxing cartoon poss?).

Is he bored? Maybe think up some new games or activies for him to do:
treasure hunt
sand box
build a fort
building blocks
coloring
finger painting

When he's out of school on vacation, can you go places like?:
a new park he's never been to?
a local fair
farmers market
lake
build an indoor fort w/blankets
buy new cheap, plastic outdoor toys: shovel, rake, bucket, toy you hook
up to hose & it sprays water for them to jump over (I think there's a hippo
one)
Build a house out of a huge cardboard box. Cut out windows & a door
have a craft day. make something HE would like or be into
buy a wood craft & have him paint it or decorate it

when he acts out, get down on his level by kneeling and ask him why are you acting like this? Is there something wrong? Do you want something? Something from mommy?

sometimes they just need the "words" to help express their emotions

try to find the motive BEHIND the outburst.

I know you're probably exhausted as a mom of two (hope I have that right?) but it will pay off in the end.

For mine, it usually means that they need a snack, drink or nap.

We can be a little too hard on 4 yr olds because most of them are walking, talking, peeing, pooping and eating without assistance, we tend to think they are little grown-ups. They arent! They've only been on the planet for 4 short years. Be patient Mom while child is trying to figure out all these things that are going through his head, the world is a complex place, so much to learn and see and do, so many expectations to meet.
This is an exceptionally sensitive age, an age that thrives on learning and challenge, their little brains are ripe and ready to suck up as much info as they can. They need lots of positive reassurance, this is the time to help them feel secure with themselves instead of timid and fearful of the world. It's truly a fun age.

Developmental changes.
Read up on the development of a 4 year old. Emotionally/cognitively/physically. Online has many articles.

4 is a hard age.
They are expected of so much... but are still very young. And if they are the oldest, they inherit, a lot of "expectations" just for being the oldest. By default. And are expected to be an "example" for the youngest. But per their own age, they are still, just 4 years old. But can have a lot on their tiny shoulders. But they may think they have to be so perfect, but cannot be. Keep expectations age appropriate. Little kids get stress too.

Kids this age get more moody when tired/overtired or hungry.
They need to graze throughout the day.

Also, teach him how to express his feelings to you. Even if that is happy OR grumpy feelings. I began teaching my kids that from 2 years old. That way they know, they CAN tell you things, and that they are heard. Not all pent up.
If my kids are grumpy, I tell them they can tell me. Then "we" try to see what is going on. Or if they just need to gel.

I would definitle try and keep track of when it is happening. If it is time for a snack...it could be his blood sugar. For some, myself included, I will literally have a mood swing that no one saw coming and it is uncontrollable. Also how old is ultra baby? With a new baby and school ending...those are major changes for the little guy. It could be he is fine while he is not thinking about it, them all of sudden it crosses his mind and he doesn't know how to react. Kids at this age are starting to realize they have different feelings but don't always know what they mean. Especially with negative ones. They don't know how to express them and release it anyway they can. Be patient with him and maybe try and think of questions that may help him express what is going on in his mind.

Kids thrive on routine and if it is just the last few Weeks of school that he has been like this give him a home routine where he will kinda know what to expect. It can vary a bit but not too much. Also make sure he eats small frequent meals and has a solid bedtime. Give positive attention and avoid negative attention.