No, you're definitely not stupid, and you definitely haven't failed as a mother. As an Occupational Therapist (and a single mum too), I have worked with autistic children, and what you are feeling as a parent/carer is very common, but you have to keep reminding yourself it's not your fault.
Autism is a difficult thing to deal with as a parent but you can get help. The first step is to get as much information as you can. Your first resource would be your local hospital or medical centre - they should have information on the medical condition itself, local support groups, therapy centres, doctors, even special schools that work with autistic children. Research the internet because your knowledge will help you understand what your child is going through, and help you to better meet his needs.
Get as much family and social support as you can. If you don't have family / friends nearby, there should be respite centres available for parents / families who need a break, so look in your local White or Yellow Pages (or directory, whatever it might be called in your country). I know sometimes you may feel guilty for asking for help, but if you don't, you will burn out quickly and this will impact on your ability to help your son.
The other very important thing you need to pay attention to is yourself. You need to try to get as much sleep as you can, eat well, and exercise - as a way of releasing your anxious energies, frustrations, etc. Looking after yourself will make sure you have the energy to look after your children.
You will also need to involve your 4year old daughter - help her to understand what is going on with her little brother, and allow her to help you because that will make her feel grown up, special and involved. She will also need her own special time with you so that she knows you love her just as much as you love and attend to her brother.
If possible, get your son's father involved. He needs to understand what his responsibilities are and his son needs him there as a positive role model. Boys (with or without autism) respond differently to men as they do women, so if the biological father is not around, try to involve another good male role model, such as an uncle, grandfather, or another male friend who loves him.
Sorry for the lengthy reply but your question moved me. You are not alone, there are many people who have been through similar situations, or who are trained to help, and they are there to support you. Just remember: You're not alone, ask for help! Take care of your beautiful little family.