What can I do to have a good MkI suitcase piano sound in my Yamaha CP-4?

You can design/build your own favorite Rhodes sound using the CP4 Stage. The original FenderRhodes Suitcase was not called the "MkI" - it was the Suitcase73 or Suitcase88 (the Mark I referred to the model that did not have the amp and speaker, as best as I can remember)... But if you are going after the early Rhodes sounds, you'll want to use the "71 Rd I" which is the model that includes the natural felt hammer Rhodes key mechanism.

Using the "71 Rd I" pre-amp as your starting point will allow you to construct the exact sound of an early Suitcase Rhodes. Most of the rest of this is personal taste.

In the CP4 Stage the "71 Rd I" is the felt-hammer Rhodes; in 1971 and after they switched to the neoprene rubber hammers (the 73 Rd I, 75 Rd I, 78 Rd II, and the Dyno all feature the rubber hammers with different pre-amp models).

Thank you very much!!