advice

I have an issue with my 4 year old son and idk where to go or who to talk to. So I'm going to post this in hopes someone has some incite for me. 
My son is a sweet, precious 4 year old. He's my whole world. 
He has behavior issues but I don't think these are normal 4 year old issues. 
His social skills are....odd to say the least. In situations with children his age he tends to wander off alone, and if and when he is near the other kids he says and does things that are weird. For instance, he barks at other kids, which causes other kids not to associate with him. Yes, barks, like a dog. He tells me he doesn't like other kids and doesn't want friends. He told me the other kids think he's a dummy. 
He's incredibly irritable and annoyed easily. Literally everything sets him off. Anything from not getting a cup of milk immediately after asking to his one year old sister wanting to play with him. Literally everything upsets him. 
He yells, screams, throws tantrums, goes bonkers more than once a day often triggered by one of the things I just mentioned. 
I've done everything as far as discipline. I've sat him in time out, I've taken away his tv, his toys, I've sat him in his room alone to think about his actions, and I've even spanked his bottom. I hate spanking. I truly don't see where hitting someone makes them a better person. That how fed up I am. 
I feel like a failure as a mom. I feel like somewhere along the line I fucked him up and idk Where or how. His first year of his life it was just me and him. When his dad finally came back in to our life we became a family. A happy family. His dad and I got married and we had a second baby. 
He's been spoiled his whole life. He's never had to go without. He's never been short on love. He is and always will be the best part of me. That child saved me in so many ways. I just want to save him. I want to help him. I just don't know what to do. I'm at my wits end with his behavior. Please, someone give me some incite as to what is causing this, what I can do. Tell me I'm not crazy. 😞