My kids are about those ages too.
My kids being 4 and 8.
My kids ADORE each other and are very close. But they fight.
From about 4 years old, a child gets more aware of things, inequities, feelings, right/wrong, fairness/unfairness etc.
It is their changing cognition....
So, from about a certain age juncture, kids fight. Even if they are the best buddies. Childhood.
My kids started 'fighting' when my son was about 3 becoming 4 years old. It was changing development. I could see it and tell.
Then the Eldest kid, gets more crafty... about 'manipulating' the youngest. Even if on purpose. So, fights happens. The 4 year old also per their age, gets MORE cognizant and will speak up more (not like when they were clueless babies or toddlers), and they WILL fight back/stand up for themselves.
So, also though, a 4 year old CANNOT 'rationalize' like the older sibling. So they cry or get frustrated.
KEEP things, and your "expectations', age appropriate. And in line with their development.
There is a BIG difference, between a 4 year old and a 7 year old or 8 year old. I TELL my Eldest child... that she CANNOT 'expect' her little brother to be like her, to act like her, to know things like her etc. Because he is 4. She is 8. So, my 'explaining' child development to her... HELPS "her".... manage better, in relation to her little brother. I TELL her, what is funny to her... is NOT to him. He understands things differently, per his age.
So, when fights occur... I step in... when it is clearly.... unfair. And when they get 'stuck' in a bad cycle and not stopping.
IF one of my kids asks for help and referring, I do it.
They need to learn... and that is what I am there for... to GUIDE... it and the fight. And doing any discipline as needed.
I ALSO, 'teach' them that they are siblings. And WE are "Family." AND that means... we ALL have each other's back. We take care... of each other and help. That they are a "TEAM".... not enemies.
all the best,
Susan