Help with 4.5 year old hitting us

Really need help with figuring out what to do with our spirited energetic and sometimes aggressive 4.5 year old daughter. Since her birth, we have always been very close. I attachment parented her, breastfed and coslept with her till 2.5. She never went to daycare, had a few trusted babysitters here and there, and started preschool at 3. She does GREAT at preschool, zero disciplinary/behavioral issues with others and she gets rave reviews for her maturity from her teachers.We absolutely do not ever hit her for discipline. We sometimes use "time-outs". In our time-outs she just has to sit down in the same room with us for about 30 seconds or until she calms down and then she can re-enter playing. She started becoming more verbally and physically aggressive with us when her little sister was born, but it's hard to know if it was just her age, or the new sibling causing the issues.The last few months have gotten particularly egregious and she has been hitting me and my husband when she is angry that she doesn't get what she wants (i.e. she wants ice cream, we say no because it's too close to bedtime, she begins freaking out, culminating in growling at us aggressively, lunging at us and hitting us). Even more upsetting for me is that sometimes we will be calmly hanging out with friends, and she will suddenly run up to one of us and hit/slap us as hard as she can across our back/legs/face, whatever she can reach. We notice that this one happens only when there is friend/company around and she does not appear to be angry or upset at all when she does this. She is usually laughing after she does this and if we ask her why she hit us, she gets angry and screams.We have tried explaining to her how much it hurts us physically and feelings-wise. We have tried giving her time-outs. We have tried removing privileges (like leaving a playdate immediately when she hits us). We have tried appealing to her 4.5 year old morals. We have tried getting her to picture herself in our shoes. We have tried pointing out how others are not hitting.I don't feel like any of this is working and I'm at my wits end. I actually find myself flinching when she is close to me and makes any sudden motions, which I think is so sad. Do we need professional help? What can we do? via /r/Parenting http://ift.tt/2atabGr