Now that I've said (hopefully clearly) which software does not exist, here are a few ideas for use of software which does exist...
1: Teensy absolutely does work as a USB device on its main USB port. You can plug Teensy into any of the USB host ports on Raspberry Pi 4, using an ordinary USB-A to USB-Micro-B cable. The Arduino IDE and Teensyduino do work on Raspberry Pi 4. Obviously Raspberry Pi is slower than most PCs, so some patience is required, but it definitely does work.
2: Linux has a feature called "USB Gadget" which causes a USB port to run in device mode. Raspberry Pi 4 has
limited support for USB Gadget mode, which is a lot more software support than exists today to cause Teensy's USB host port to run in device mode. If you try this path, I would recommend first using USB Gadget mode with the Raspberry Pi connected to one of your PC's host ports. You will have 2 challenges, first just making USB Gadget at all so your PC detects it as a USB device, and then also configuring USB Gadget to act as a USB disk drive (USB mass storage class). How to make USB Gadget work as a disk drive, I do not know. Hopefully Google can lead you to that info, if it exists. Again, get this working so your PC is able to see a USB disk drive appear. Only after you are certain USB Gadget is working and it really is a USB disk, then try connecting to Teensy USB host port and run examples like
File > Examples > USBHost_t36 > Storage > ListFiles to access the Raspberry Pi USB Gadget disk drive.