sorry for the time lapse since you had this. Considering your just a weekend dad, instead of coming up with your own rules, and you and your ex are getting along fairly well, perhaps you should consider doing what she does, or seek her advice telling her the boy's acting up, etc. and you're unsure how she's disciplining him. By no account, you should EVER spank a child.
Not to mention if you're spanking him on only weekends when you're seeing him, he's going to freak out and poss. not want to go to daddy's for fear of being spanked, thus make your ex think you're abusing the boy.
constructive talking, placing time outs, taking away toys, no tv etc. is the real way to go on that.
there's a book I read called "raising confident boys" I'd suggest and parent read as well.

First, Welcome to the world of parenting a 4 year old. I am on my 4 years old for the 4th time and each kid has been different. It is normal for kids to assert themselves and push boundaries. Especially in a divorce situation.

That being said I want to echo what JTGPP said. If you and your EX get along, work out a method that works for you both to discipline you son. Also do your best to follow the same rules, routines and discipline that she uses. It is too confusing to a child that age to have different rules at Mom's and at Dad's.

Finally I want to say, though I don not spank my kids, I do not see a problem with spanking. I qwas spanked as a child, and I feel as though I am fine for it. I will caviat that with saying I was never spanked in anger, and it was always warned first, then executed and then explained why I was spanked. I think that society has done itself a great disservice by making spanking as a form of discuipline such a taboo, and I will probably get stoned for this opinion. I could go on about this, but that is not what this thread is about.

The point is get on the same page as your ex as far routines and discipline and be consistant. That is what you boy needs.

THANKS FOR THE ADVISE. IT WORKED FAIRLY WELL FOR THE FIRST TRIAL RUN. THE 1-2-3 GOT IMMIDATE RESULTS, AND THE TIME OUT FOR A BIT WORKED WELL, AND ALLOWED ME TO KEEP MY COOL TOO...OOH, AND THE TAKING HIS VIDEO GAMES AWAY FOR BAD BEHAVIOR WORKED GREAT TOO.
THANKS AGAIN.

my nieces are four an wen they play up we put them on the naughty step and tell them wot they did wrong n then go back after a few minutes n they apologise its like a time out thing. my nieces r pushin their luck now there 4 too i think its just a phase where they are tryin to test u

He's definitely testing you. As was said talk to the ex so you are at least disciplining in the same fashion. I've found that corner time and taking things away are effective. Try not to raise your voice (i.e. yelling) talk in a stern manner. Explain why what he did was wrong, and why he is being punished. After the punishment has been completed talk about ways to not have the same issue again. Not to get on the band wagon, but I was spanked as a last resort. If you have to spank, make it just that, a last resort.

A method that has worked for me recently was the creation of a chore chart for my two kids (ages 5 and 7). It consists of little things appropriate for their age that they can do easily like practicing their writing and reading skills and keeping their rooms picked up, etc. Each chore has an assigned monetary value $0.05-$0.15 per day. It is something that their mother and I agreed about and both put into effect. Now, aside from the obvious benefits of making an allowance mean something and teaching the value of a dollar we have started to use demerits to control temper tantrums and bad behavior. Nothing has shut my kids' bad behavior down faster than letting them know that a quarter will be taken from their allowance if they continue to misbehave. I do make sure to explain to them how this relates to real world experience, ie; marked poor performance at my job could mean no raise/bonus when it comes time for my review. Eventually they come to learn that good behavior benefits them.