I have no children of my own but have heard that rear facing is the safest. I haven't looked into it much but have heard the same that has been mentioned.
I think most parents change to forward facing because it is easier to get them in and out and usually more comfortable for the child. It may or may not be the safest though. I have only had a couple who I had rear facing and that was until they were about 12-14 months. When the parents changed them to forward facing, so did I. I now have them all as forward facing when we do go out which is rare now compared to when I first opened.
I haven't heard of a 4 year old in a rear facing seat but if she still meets the requirements for the seat than I see nothing wrong with it.