There is basically no point to add the RAM chip if you're using Teensy 4.0. The only supported chip adds 128K of RAM. Teensy 4.0 has 1024K internally. Best to just use Teensy's RAM, which is much faster and more efficient than SPI for the external chip.
But if you do add the RAM chip, the only supported effect is delay. You could use delay to build echos. So maybe that could be useful, but I would suggest you first experiment using Teensy's RAM.