Is the second generation keyboard folio for the iPad Pro 2nd compatible with the new iPad Pro 4th generation?

See the compatibility section of:

Smart Keyboard Folio for iPad Pro 11-inch (4th generation) and iPad Air (5th generation) - US English - Apple

No.

The three-pin Smart Connector that electrically connects the iPad Pro with the folio keyboard is located differently between the two models of iPad Pro that you cite.

As such, the folio keyboard that is compatible with your second generation iPad Pro 12.9” cannot be used with the fourth generation iPad Pro 12.9”.

Thank you for the clarification.

The folio keyboard designed for the fourth generation iPad Pro 11” (as mentioned by Illaass), is backwards compatible with the earlier second generation iPad Pro 11”. However, it is not necessarily true that the folio keyboard purchased with the earlier version of the iPad Pro 11” will be forwards compatible with the new fifth generation iPad Pro 11”.

Apple only quotes compatibility of model accessories within its current sales catalogue.

The folio and Magic keyboards both have a rear aperture to accommodate the iPad’s rear camera array. While the overall dimensions of the iPad are unchanged, the camera array “bump” sometimes gets a little bigger in successive models - requiring an enlarged aperture in the keyboard backplate to accommodate the iPad’s rear camera.

For this physical reason alone, newer keyboards are usually backwards compatible - as a smaller camera “bump” will fit within a larger aperture, but older models of the keyboard cannot necessarily be used with newer versions of the equivalent iPad. A larger camera “bump” will not fit within a too-small aperture; even a sight mis-fit will affect the correct alignment and contact of the Smart Connector that couples the iPad with the keyboard.

As such, until such time as the fifth generation iPad Pro 11” arrives and its “fit” tested with the older model keyboard, it is impossible to provide a definitive answer as to whether or not you will be able to use an older folio keyboard with the new iPad Pro 11”.

If contemplating purchase of the new model, you might only discover the need to replace an existing/older keyboard when you attempt to couple the two items. If all is well, you’ve saved some money; if not, perhaps you’ll contemplate the newer keyboard.

To summarise, until the new iPad reaches the hands of those that can test them, the answers to some questions cannot be be provided with certainty. Hopefully this clarification and explanation will prove to helpful.