Anyone have to tell a young child that you are now daddy rather then mommy? Was it confusing for them, or do they get it. My son seems to be getting it, but I feel slight guilt since his bio dad passes away when he was 16 Mos old. He is now four and when I told him I was now going to be his daddy rather than his mommy. Maybe it's just confusing for adults, not children...
It's easy to tell young kids. It is about the same and saying it is sunny or rainy out. They just accept it and move on. Four is a great age to tell him as it can get harder the older they get. For example some teenagers may feel embarrassed when a parent is at the start of their transition due to peer pressure. If he is fine with you being daddy now and is calling you daddy I would not worry about it.
I am a step parent (though very very involved with my kids). They were 11, 8, and 6 when we told them I would be transitioning. They all handled it very very well and we have had no issues. In fact our oldest said "huh... Yeah that makes a lot of sense".
Kids usually have an easy time with it, especially young kids. Making the switch now eliminates a lot of the confusion with friends and other parents as your son probably hasn't really started school yet.
So, yes, I think it's easier for kids to understand as they do not have years of societal imprinting telling them what is "different".