Why is my 4 yr old so defiant?

My son is turning 4 next month and although he was a very sweet easy going baby he has changed drastically since then.  Starting around age 1 he began to display defiant behavior.  It started off in small doses and has gradually come to where I'm at now.  My son has been in preschool for about 4 months now and hasn't had any problems, as a matter of fact his teachers LOVE him.  He gets along with all of the other children and is well behaved.  His teachers say he's very smart but has NO attention span (this I already knew).  However at home he is a full flegged MONSTER.  He is mean, says and does mean things, he sticks his tongue at me when I am trying to talk to or discipline him, he throws multiple uncontrollable fits a day, he does not listen, he throws toys (at people, our animals or whatever else), he pulls on our animals' tails, he hits them, chases them, scares them and jumps or lays on them, he mocks me when I'm talking to him, he runs from me and thinks it's funny (most of the things he does he thinks is funny), he is potty trained but has recently been pooping in his underwear, he screams, shouts and is CONSTANTLY testing me.  I will ask him not to do something and tell him that he is going to get hurt and he will smile and say 'No I'm not!'.  I admit I am far from a quiet person and probably raise my voice when it's not necessary so I can understand why he may scream or shout but I'm not understanding the constant defiance!  He will be on his time-out chair and continually stand up, or move the chair inch-by-inch or play with ANYTHING within reach (and he will stretch to reach it).  I will tell him that he needs to stay seated in his chair and he will stay standing up until I get within reach then he'll sit back down really fast.  Since he knows he's suppose to remain seated but wants to reach a toy (or other object) accross the room he will walk with his bottom still on the chair to where he wants to get and then put the chair back down (by this point I've got steam coming from my ears).  Sometimes I don't know wether to laugh because he's so flipping clever or cry because I'm so at my wits end!!!  He's mostly to the point where he'll sit for his time-outs now however while they may stop him at that exact moment they prevent nothing even when threatened with one.  He is the ONLY kid I know that doesn't watch T.V. (not commercials, not movies, not even cartoons).  He literally goes from one thing to another in a matter of (maybe) 2 to 3 minutes.  I've tried everything I can think of ( from different disciplines to different rewards).  To put it mildly...my son is out of control...HELP!!!

[MotherlyQuestion]

have you taken him to doctor, what if he is going through a chemical imbalance, hope not, but sounds like he is just going through a face, or he might need to see you act completely different than you normally do,  like my situation, my three year old has been displaying very bossy attitudes, and says no to almost anything i say, and also tells me what to do, i hope is only temporary