ignore the 'threats' for now, and concentrate on the fantastic fact that they have found and funded a ft TA for her. you need to arrange a meeting as soon as possible, because the more that the TA knows and understands about dd, the less likely it is that any exclusion will happen.
does she have specific triggers? (especially in a classroom situation?) is there a warning period - does her beahviour escalate, or is it a really short flash-to-bang period?
the TA needs to know how you deal with dd at home and which techniques have the best results. does 'talking her down' calmly work, or does she need training in restraint techniques?
what is the current classroom plan in case dd gets overwhelmed? (most classes have a standard procedure where one child runs to the office to get help, the rest of the class evacuate to the school hall/ the next door classroom/ and the teacher/ TA remains with dd until the situation is under control and it is safe for the class to be allowed back in/ or for dd to have been removed for quiet time to the library/ safe space etc.)
have they implemented a system where she can 'opt out' in a non verbal manner if she needs to? some teachers have special cards whihc children can show, or a code (they put their pencil/ red tractor/ ruler/ wahtever on the edge of the desk and leave to a pre-determined safe zone) this will be easier with a TA providing support and watching for clues/ triggers.
hopefully dd will 'click' with the TA. but you need to make an appointment with the ht and the Ta and classroom teacher an focus positively on how this can work for the best. it's a great thing. quite why the ht had to ruin it by threats makes my blood boil, but try and get past that. he's probably feeling emasculated by the fact that he can't control a 4yo girl. which is his problem, really.
none of that stuff is completely out of the ordinary and schools do manange to cope - with the TA there is every chance this will be the turning point that matters.
and if it isn't, then it was never going to be the right setting for her, and so it makes sense to walk away in a dignified manner and find somewhere that suits her better (whether ms or no)
good luck x