My son has never known or meet his real father he thinks my husband is his dad. He is now 4 years old and going to school and he has a different last name then us and I want to change it. Will I need the consent of his real father even though its been 4 year and no contact with him and we don't even know where he is?

Yes you will need his consent to change the last name legally.

You can still petition through the courts to have the right to change your child's last name without his consent, you will just have to go to court and prove abandonment, and the judge will have to deem it in the best interest of the child to change the last name... it will be ultimately up to the courts.