My son was just like that at four. He still is at eight. When we started looking for help, we were told they can't offically diagnose a child until at least age seven, but they knew then something wasn't right. On top of being overly moody, hyper, argumentative, late sleeper/early riser, and unable to follow direction, he also had bathroom issues. It was utter Hell.
We tried for two years to use behavioral changes instead of meds, but when he was 6 1/2 something happened that just messed that up. He finally had to go on meds in Jan 2006 at age seven. There's a slight difference, but not much on my end. The school has been amazing, and if he's in pre-k and having problems, check if the county can help.
Make a journal of your son's behavior and his mood swings. The doctors ask you how many times he does this and that, a journal will make answering easier. This will also aid in charting his mood swings, which could indicate a more serious problem like bipolar disorder. Many kids diagnosed as ADHD turn out bipolar in adulthood. Don't stress about it, but it's something you may have to deal with.
I know you're having a very hard time right now, so try not to feel guilty about your feelings conflicting with your love of him. I've done nothing but try to help my son for four years, I fought pure evil for him, but there are days I go hide in the bedroom because he's being a pain. It doesn't make you a bad mom, so please don't think that.
Talk with your pediatrician and if he/she says he needs more testing, get them to refer you to a good psychologist or psychiatrist. No official diagnoses will happen, but they'll at least be able to help. Don't try to diagnose him on your own, though. And be very careful with whom you go to see about him. I hate to sound mysterious, but there are some therapists out there that could do more harm than good. I met one, trusted her, and she almost destroyed my family.
Now for the scary part. This will be very hard, and there's going to be days you'll want to scream, so if you can get a support group do so. I didn't and it caused me to go into deep depression. Take the help from anyone, especially with two other kids. Arm yourself with knowledge and it will make the ride much easier.
Good luck, J., and if you're near Alpharetta I have a couple of good docs for you.