I am so, so, sorry for your situation.
You sound like a very good person, that quite frankly is in over your head.
Your brother is acting out because he misses his parents, and doesn't understand the new situation he is in. It is impossible to explain this to a 4 year old. At 20, with a pregnant girlfriend and a 13 and 9 year old kids to be responsible for, I am afraid you are probably constantly stressed, and not in the best emotional or psychological mood to be dealing with a greaving 4 year old.
You need professional help. Please try to find a good Christian church that can advise you. Or look in the yellow pages for social services. Dig up phone numbers of an aunt or uncle. Honestly, a church might be your best bet, a better relationship with God wouldn't hurt.
But please, don't think you need to do this all on your own. You are young, and have your own life ahead of you. Your siblings deserve mature parents raising them, and you deserve a break, plus you have your own responsibilities now with a child. You are a good brother and a good son, you are trying your best. But sometimes doing the right thing means understanding your limitations, and asking someone else to help you out.
Good luck, and I will be praying for your and your family.