Its not untypical behavior for a 4 year old. Read all the "out of control 3,4 or 5 year old" posts on this site. And yes, he probably is testing you, but not to see if you are going to leave him. If he is a sharp kid, he is just stretching his boundaries and sounds like he is winning the real estate game.
Some suggestions. First you need more information about effective discipline, I recommend "SOS Help for Parents," by Lynn Clark. Its about how to effectively use the timeout method. I've found a lot of parents just don't use it correctly.
Second you really need more info about ADHD, so that you know what to expect and how to deal with it. On the ADHD forum I recommend "The ADD/ ADHD Answer book," by Susan Ashley a lot. I think that it would be very appropriate in this case too.
He is also of the age that you can start reading to him from several sets of really wonderful books aimed at the 4 to 7 year old crowd. Besides being a great bed time thing, the messages are right on. A good start can be found here and then look at the other books listed below for what you thing might be appropriate for him. (each time you click on a particular book, you will get more ideas below) http://www.amazon.com/Hands-Hitting-Ages-Best-Behavior/dp/1575420775
Hope this helps!!!
Firstly it may be a good idea to have the meds checked out, can you tell us which he is taking , you may need the dose adjusting or the med changing, they can have side effects .It is possible he is missing his dad , has he any siblings?