Teensy 4.1 features the fastest microcontroller, an ARM Cortex-M7 processor at 600 MHz, with an NXP iMXRT1062 chip, the fastest microcontroller available today. It features and an expanded set of powerful peripherals in a 2.4 by 0.7 inch form factor.
Teensy 4.1 comes with four times larger flash memory than the 4.0, the bottom side of Teensy 4.1 has locations to solder 2 memory chips.
The smaller location is meant for a PSRAM chip. The larger location is intended for flash memory.
Teensy 4.1's USB Host port allow you to connect USB devices, like keyboards and MIDI musical instruments. A 5 pin header and a USB Host cable are needed to be able to plug in a USB device.
USB hubs may be used if more than 1 device is needed. See the USBHost_t36 examples for details.