Personally i wouldnt second guess yourself. if that's what it took for him to understand than so be it. a 4 yo cannot even remotely understand addiction so you have to keep it to terms that they do understand and sick is a word they understand. he's probably seen dad act "weird" and you can explain that the things daddy puts in his body makes him act weird and do bad things.
There are also age appropriate books that you can get too that help explain this. I know when my daughter was younger she was given a book from a counselor on death when her cousin died - it had lots of information but also activities for her to do to help her understand. these books are kind of hard to find but they are out there - she still has it and it means a lot to her.
one site i saw says this:
Toddlers and preschool children understand simple, short sentences. They need concrete information and not too much technical language. Explain the problem simply and then try to make the child’s life as normal as possible. After explaining the problem, engage the child in a fun activity.
School-aged children can handle more information than younger children. They might already have had drug information sessions at school. Be prepared to answer their questions honestly.