Really, a 4.5 yr old wants specific details and isn't satisfied with, "Daddy helped put the baby there?" I have a 4 year old and I'm pregnant with my 4th child and he's never asked for specific details. I guess every child is different.
I wouldn't give more details to a 4 year old. How in the world would a child of that age understand the details of intercourse anyway? I can't imagine so your explanation is enough, in my opinion.
This is what I've told my children: "When a mommy and daddy love each other very much, God allows them to have a baby." My oldest child is 11 and has asked, even my 8 year old has asked for more information and I have said that mommies and daddies have special private time together and God allows them to make a baby together." They were satisfied with that and I feel that is more than enough info for their age level.
As for how the baby comes out, all of my children, including my youngest know that a baby can come out two different ways: vaginal and c-section. They know this because I have only had c-sections and I watch these shows called A Baby Story and they have seen a few episodes with me and they have also seen women "in position" for a vaginal delivery so the topic came up, "What are they doing?" and I explained very matter of fact that a God designed a woman's body to deliver a baby from the vagina but some women, like their mommy, cannot deliver a baby that way and a c-section is the only way to deliver a baby. My children also know about breastfeeding, as my two oldest remember me breastfeeding my youngest, and they know I intend to breastfeed the baby I am currently pregnant with.
Honesty is always best, don't tell fake stories that some bird dropped a baby on your doorstep. But, a 4.5 yr old doesn't need more details as I cannot imagine what details they would expect.