Hi srockrae,
Your 4 poster bed should have 5 wooden cross slats with a center rail and good center support to the floor (in a king size preferably with two legs) to provide suitable support for a foundation.
I’m not quite sure which type of foundation you have but if it’s one of the wire grid type like this you can see my thoughts about them with an all latex mattress in post #10 here.
You could add slats on top the way you are suggesting with a suitable spacing if the top surface of your foundation is flat and evenly supportive (with a contact point on each side and in the middle) so they don’t tilt or bend and you have a way to secure them to the foundation to keep them from shifting. A bed rug would probably also work well.
Phoenix