Who to turn to when your own parenting clearly isn�t enough? Horror DD 4.5yo

[NotAnotherNewNappy]

DD2 is 4.5yo. She still gets v angry tantrums like a 2 year old when things don?t go her way. She is increasingly violent and nasty. Hitting, biting, spitting, bullying, making threats, name calling etc.
What can we do? We try time out, sanctions, taking privileges away etc. We try ignoring her, calming her down, giving in to her. None of it seems to work. We?ve just spent 2 weeks on holiday and she was horrid ? terrorising DD1 (7yo) in the back of the car.

She starts school in 2 weeks ? I?m terrified of how she is going to behave. What can we do? Who can help us?

Your health visitor should still be able to help also when she starts school her teachers should be able to help

She sounds very much like my DS.
It's not really a teacher's role, but I asked for help from the home/school liaison worker.
If it's any consolation, he didn't take the behaviours in to school. He did, however, make getting there on time pretty much impossible, so this is why I was offered the help.
Really, she just suggested 123 Magic and stuff I'd already tried or discounted.
The real help came from a plea I posted on here, in desperation.
One year on, things are much calmer. He still has his moments, and the holiday has been hard work.
How does she sleep? I've realised that lack of quality sleep was our main problem.
We had a really bad day a week ago, and I considered contacting my GP. I'm not sure what they can offer, though.
Not much help, I'm afraid

DS2 is 4y 4m, so just a tad younger. He has recently taken to epic tantrums. Mainly about not being able to keep up with his big brother.... Two things seem to help. Enforcing occasional naps by having a half decent car journey shortly after lunch, and more attention when he's being lovely. So reinforcing the desired, and ignoring, or focusing on his brother when he's horrid.

We are both pretty calm - sometimes I shout. DD1 is v calm and never violent. She is def worse when she is tired so I try to make sure she has a good night's sleep - she's never really napped.

I am so rubbish at this name change business...