I have photographed mammals and birds with both lenses and would like to help you with your purchase decision.
You can buy both lenses and accessories here (affiliate links):
Canon RF 600mm f/4 L IS https://bhpho.to/48YNuHT
Canon RF 400mm f/2.8 L IS https://adorama.rfvk.net/jrxqnP
Canon RF 1.4x Extender https://bhpho.to/3RW1cWj
Canon RF 2x Extender https://bhpho.to/3GY9YNs
Tripod Foot https://bhpho.to/3vGlvyt
LensCoat RF400/2.8 https://bhpho.to/4bRiew6
LensCoat RF600/4 https://bhpho.to/4buaMXO