Lenin "But he only stopped spitting when we completely stopped reacting to it, at all, ever. Then he stopped niggling DS2 when we rewarded him immediately and unexpectedly for playing nicely with him."
Well there you have it - ABA in a nutshell 
I wasn't suggesting that a child with asd would never grow out of any behaviours naturally. But if a certain behaviour (eg aggression) is continuing way beyond an nt child of the same age, and not responding to the normal techniques applied to nt children (eg saying no, punishments, sanctions, time-out, whatever), specific, tailored intervention has to be applied. I would say that a 4yo hitting parents is beyond the normal expectation for her peers, and she will definitely require some extra intervention to stop it. If she's got this far with no consequences for it, she's learnt that it's a pretty effective way of getting what she wants, so of course she's going to continue.
oodles sorry I don't know too much about Hanen but have read lots of great-sounding stuff about it on this board (in particular, More Than Words seems highly recommended).
"Could therapy help him to have a real give/take conversation about stuff in the real world?" yes yes yes
ABA definitely used for verbal children (my ds has been verbal since a year old), and appropriate communication can absolutely be 'taught' through ABA - we've spent a large amount of our time doing just this with our programme (Verbal Behaviour strand of ABA). In fact, our consultant told us that the kind of echololia you describe is a great sign, and shows that a child has a real, innate desire to communicate (a great starting point), so just needs to be taught the skills how to communicate appopriately.
The cynic in me would suggest that you don't waste any more time waiting for helpful or useful provision from the paed/neurologist/salt etc. You will find 10 times more useful practical help, support and info from boards like this one 