Out of control child with history of bad parenting

That's right, let me start off by saying flat out I messed up... too much yelling, and when that didn't work ignoring behavior in my child in the past 3 years.  Then I woke up and realized I HAD to change something FAST, and have been attempting to be a much better mother... but I don't know what to do to correct my son's behaviors, or whether he needs other help as well.
My son will be four in about a month, and I have struggled from the time he was 2 1/2 to get these behaviors under control: screaming, kicking, hitting, biting, out of control tantrums every time he does not get his way, (and sometimes when he does), and a general attitude of not caring what anyone says or does unless it helps him.  He will even tell you he doesn't care if something hurts, offends, or breaks you or something you own.  NO disipline has worked... time out (you would literally have to sit on him to get him to stay, we've had battles that lasted over 6 hours before), spanking (he hits/bites/screams/throws things back at you, even if he wasn't before the spanking), taking a toy away, denying what he wants to do when bad behavior is present... and he also will NOT sit still, look at you, or talk to you when he's "bad" (I have attempted to explain things to him nicely before many times.  He will not talk to you...)
I am unsure what to do- nothing works.  People tell me that's just normal for a kid until they spend a day with him... then they (literally, these are things people have actually called him recently) describe him as "a monster" "Tarzan" "a demon child" "out of control" "psychotic"... and I'm so afraid they might turn out to be right if I can't get the situation under control!  If you can help in any way, please do!  I will try anything!

[Kevin Kennedy, Ph.D.]

This situation is well beyond what you can handle by yourself. It is to your credit that you are recognizing your inadequate parenting. But you have established a poor foundation for your child and now you are experiencing the results of that. You have lost your jurisdiction over your son. Arrange an evaluation with a child psychiatrist.

[avienndhra]

ps- I have noticed he gets worse before he falls asleep... so it's either out of control, or sleeping.  He also sleeps more than most kids.