Teensy 4.1

You haven't indicated how you intend to interface the W25X40 to your T41 (external SPI socket/adapter, etc. ??), but maybe the "SPIFlash_ReadWrite" example sketch which is available from the Arduino IDE might have some helpful info . . .

Mark J Culross
I was hoping that there was already a library that directly addresses that series of chips. The interface will be through an external socket or pogo pins on a test fixture.
Mark's answer is indeed the library you want.

In Arduino IDE, first select Teensy in Tools > Boards. Other menus update depending on the selected board.

Then click File > Examples > SPIFlash > SPIFlash_ReadWrite.

Libraries LittleFS and SerialFlash also access this type of chip, but those libraries emulate a filesystem. The SPIFlash library is meant for raw access directly to the chip without any filesystem layer.