Except for the specific pins where you connect the 3 signals, this is identical on Teensy 4.0 and 3.2. So please keep in mind anything you find about Teensy 3.x applies to Teensy 4.0 too. They work the same way. For this particular use, the only difference is which pins get the I2S signals. Both are shown on the I2S input documentation in the design tool (right side panel, scroll down to hawrdware).
https://www.pjrc.com/teensy/gui/?info=AudioInputI2S
You'll see one column says "T3.X Pin" and the other is "T4.X pin". This is the sort of documentation published for Teensy 4.0. When everything else is the same for all supported boards, the documentation doesn't specifically mention Teensy 4.0. In places like thing pinouts, the boards are mentioned.
like a .read() function or similar to get the current microphone value?
That's not how it works. Reading each sample and trying to keep up at 44100 Hz while also doing other work is extremely difficult. The audio library is designed to let you easily build reliable processing that keeps up with the audio rate even if your program does other work. The high speed processing happens entire within the library.
Check out this 31 page tutorial. Turn to page 24 for how to do FFT analysis. You will probably need to read pages 8-11 first to learn how the design tool works.
Again, don't let the mention of Teensy 3.x frighten you. Teensy 4.0 did not exist when this tutorial was written. But again, except for the specific pins, it all applies to Teensy 4.0. The audio library works the same way.
Also in the tutorial you will see the audio shield in every example. The use a MEMS mic, just connect it to the I2S pins instead of the audio shield, and delete any code which configures the SGTL5000 chips. Those MEMS mics don't use any config, as they have no settings or options to control. You just turn on the I2S hardware and they transmit data.
While it might be overkill, you might click File > Examples > Audio > HardwareTesting > Microphones > SPH0645 to see an example for one of the hardest-to-use MEMS mics. Most of them don't have the DC offset issue of SPH0645, so you probably don't need that filter and maybe not the amp either.