Could be a food safety disaster waiting to happen, or could be fine. You can rent combo (heat holding storage bins), chafing dishes and other heating options; look into it now.
You only have 4 hours off the heat to transport and serve the turkeys.
For 400 people, you only need 400 pounds whole turkey; 300 if many are children.
You need to specify that the meat is delivered sliced and ready to serve; it is unsafe to reheat whole turkeys.
There is a lot of useful info on this size undertaking, including safe reheating of turkey, in the holiday cooking section at the top of big pots. Read and then write back.