How to make my 4 1/2 yr son understand of why I work/overtime

I suspect that the most important thing your son wants to hear as a response is That I spend a lot of time working is not a rejection of you; I love you and I cherish the time we spent together.

That said, one important reason why people work is to be able to buy stuff is a basic fact about the how the world works. The question of how to chose a fulfilling career is not going to be accessible to your son yet. It is probably a good idea to deemphasize the role of buying toys in particular, otherwise you may be asked to deliver on that aspect more often that you'd like, and I could imagine that this could constitute an unwanted push towards a more materialistic worldview.

Your son may or may not have any interest in what parent is working means beyond the first paragraph. If he does care, and your work is something you can explain, you can of course try to explain the importance of your work a bit. But be careful with the first part here - it would not be unexpected of a four year old to hear I work as a pediatrician to cure these kids as parent loves these strange kids more than me. In a few years, he will be better equipped to understand concerns beyond is own daily reality.