Hi marilyno,
SleepEz is a great choice and it would certainly fit your criteria :).
Since a latex mattress generally does best with a rigid, even, and supportive foundation … and since your box spring is over 20 years old … I’d suggest that you will need a new foundation.
You have several options that you could use for your 4 poster. If it has a center support and you can add slats directly on it (minimum 3" apart and preferably less than that) … then that would work well but it would be quite low. You can also add either a wooden slatted or a wire grid foundation (similar to your boxspring but more solid and rigid) onto it that is a similar height to your current boxspring if you prefer or need more height. I personally believe that a slatted wood foundation or strong wire grid foundation is much preferable to a solid platform foundation because it allows the mattress to breathe and ventilate moisture. Make sure too that you have center support down to the floor on a king size bed.
There are lots more options for a wide variety of foundations from very inexpensive to much more expensive for your mattress in post #7 and onwards in this thread, along with this thread and post #47 in this thread.
Most manufacturers including SleepEz will sell a foundation that is suitable for the mattress you are buying.
Phoenix