My son has a REALLY hard time understanding consequences, discipline, and abstract thinking. Wanting some input.

My son just turned 4 this month. He doesn't think like other kids his age. I started to notice it about a year ago. I thought he'd grow out of it and now he's getting older and it's not getting better. His preschool confirmed my suspicion and I had him evaluated last October. They didn't detect any behavioral problems. No autism or ADD. After what happened today I'm going to call his pediatrician on Monday, but I'm hoping there are other parents who have been in similar situations and that can maybe direct me in the right direction as far as some research or some sort of diagnosis. Obviously it won't be an official diagnosis, I just like to do my research and I don't even know where to begin.

The way I usually describe it is that he doesn't understand abstract concepts. If I ask him "what if..." his answers are strange. I ask him LOTS of why questions to get his brain thinking. He is starting to answer them more correctly, but still has a hard time grasping the concept.

As far as discipline goes we have learned that when we yell he does not pay attention to what we say he only focuses on the fact that we are yelling. Putting him in time out is the same. We have found the most effective thing is to stop and calmly ask him questions. When he answers the questions we know he's understanding. We want him to come to conclusions and not just be told them (but if he doesn't say the right answer I tell him the correct one, talk about it and ask again). He just does not respond to discipline at all. His preschool says the same thing.

So here is what happened today that is the last straw (and because of people that are inevitably going to tell me how horrible of a parent I am this is a throwaway. If you read carefully you can understand that this was not us being neglectful so please read and don't jump to conclusions). This morning my dad stopped by. He was on a bike ride and ended up by our house and wanted to say hi. We put a child lock on the door and moved it obviously so I could open the door for my dad and when I shut the door I forgot to put it back on. When he left my son either wanted to talk to him, or wanted him to come back, or was in some way concerned that he left (not sure) and after my husband and I went back upstairs (we live in a split entry house) my son went downstairs to put his shirt on and ran out the door after my dad (about 2 minutes after he left). My son goes out the front door all the time. We CONSTANTLY tell him not to because it's not fenced like our backyard. He does. not. listen. No matter how many times we tell him why we don't like it, he just does it anyway. And you have to understand, it is NEVER to be malicious towards us. He is the absolute SWEETEST boy you'd ever meet. He is not devious in any way. He does it because he wants to and doesn't see why it is wrong. He just wants to do it so he does. So usually he doesn't leave the front yard, he'll just go play in the dirt or look for bugs, just do things that a normal 4 year old boy likes to do. So after about 5 minutes my husband went to go get him and he was gone. He frantically searched every room in the house, the backyard, etc. After probably 1 minute of searching came into the bedroom where I was sleeping (I worked really late last night and had to work a couple hours early this morning) and told me he couldn't find him. I immediately got in the car and started driving around the neighborhood. I found him within 5 minutes on the busy-ish street that lines our back yard. Someone had stopped and had just dialed the police when I found him. I'm not even kidding, this was TEN minutes after he had walked out the front door. This is not the stories you hear where several hours later the parent realizes her child is missing. We were SO upset worrying about him. And tried and tried to explain it to him. I'll ask him questions like "what if the stranger took you and hurt you" and he'd answer "the stranger didn't hurt me." Answers like "ok yeah I know that's not the point." We explained all scenarios that could have happened. It was not a fun day for us.

So I'm not really looking for discipline techniques. I've tried everything and have found what works the best for him right now. What I would prefer is if you have kids that have had the same problems understanding these types of things. I would do better understanding how his brain works so that my husband and I can develop a parenting style that caters to his needs. Thank you for reading if you got this far I really appreciate it!

Don't get me wrong, my son is very smart. He can read. He is really really good at memorization. Just not thinking outside the box.

TL;DR My son has a really hard time understanding natural consequences of his actions, responding to discipline and understanding abstract concepts such as why or what if (hypothetical) questions. Have you had children with the same difficulties? What are some good resources, or maybe their disorder if applicable?