As other posters have said, you need to frame this as what is in the best interests of your DD. So if your Dd is unsettled by the two day schedule, and who would not be, then it makes sense to try 4 with you, three with him, and see how that goes. I honestly think a mediator will see that. But then if she struggles with that, then EOW and one day a week.
You are only four weeks out of the split, so it is all very raw and emotional.
The other thing which I think is what does DD want? Where I am, the wishes of the child would have some weight. However, if your ex is not willing to listen to DD, then it would need to go to court for her to be consulted, and I don’t know how that works where you are.
But anyway, the main thing is that when you go to mediation, you need to frame it as what is in your DD’s best interests. It is best to have a relationship with both parents and spend time with both parents, but it needs to be safe and manageable for her. If you have been the primary carer, of course she will struggle going to a 50/50 arrangement. So you are not saying no, you are trying to work out a pattern which will work well for her.