My four year old is out of control

My son is about 4 and 1/2 he starts pre-school in about 2 weeks but has been in an at home daycare with about 6 other kids for almost a year.  I started working a full time job last year and his behavior has been steadily going down hill since then.   He has an older half brother that my husband and I have residential custody of who is 8 and we are in the process of having tested for ADHD.  There is one other child in the house, our two year old daughter, who has recently started the smacking phase when she doesn't get her way.

My question is - over the last few months my 4 year old has steadily increased his miss behavior.  Out righ talking back, telling me I'm not gonna do it, you don't love me anymore, yelling at me and his father -not to mention any adult who trys to correct him, ignoring people when they speak to him, and now we have reached a new peak hitting.  And I don't mean the playful smack I'm frsutrated/tired kind of hitting, I mean taking his arm back and smacking people as hard has he can usually other children.  Today was the final straw for the woman who runs the daycare he is at.  Today he was hitting everyone, she would place him in time out and then let him up and he'd get up and go hit the next one, I had to have my husband go and pick him up and take him home.  I'm not sure what to do.  I'm out of ideas.  I've tried time out, I've tried making him clean things up (aka - picking up all the rocks or sticks out of the yard while his siblings get to play), I've tried talking with him - asking him why or telling him use your words.  NOTHING works.  I'm at my wits ed as to what to do to get him back under control.  Does anyone have any ideas?  Please help!