I also have a 4.5 year old, he also seems to get very angry sometimes - he hits out, hits or bites his brother (3yo), is defiant and rude. He says he doesn't love me, doesn't want a mummy or a brother, wants us to go away, doesn't like us etc etc
I think this is reasonably normal for their age - I'm guessing she has just started school? Has she settled ok there? Tiredness and hunger always seem to make things worse here. In my opinion doing extra activities on top of school is probably too much.
I would stop any after school clubs/activities for now, do earlier bedtime and keep an eye on what she's eating at lunch. My son is a mega fussy eater so I do him a packed lunch and take a snack for straight after school too.
The thing I've found to have the most impact has been kind of love-bombing him though. Me and his dad have both made more effort to chat and spend quality time with him on his own and praise, praise, praise good behaviour.
We also try to enforce just a few rules - being kind and listening to instructions are the main ones. I found asking ds to 'be good' was way too abstract, he needs specifics - e.g. " While we are walking ds, please stay beside the pram and hold my hand when we are crossing the road"
I realised I felt like I was constantly criticising him at times so now I try to ignore most stuff and really reinforce good behaviour.
It's most definitely the most challenging thing ive had to deal with so far as a parent, we aren't perfect and its baby steps but things are getting better slowly...