Thanks. I believe the 3.3v output is where the problem lies since this is a 5v module.The I/O pins on the Teensy 4 are not 5VDC tolerant as inputs (3.3VDC max input level without causing damage), & only put out a nominal 3.3VDC as outputs. Most of the "Arduino compatible" relays require a 5VDC level signal to drive them, so you'll probably need something (external driver/adapter, transistor level-shifter, etc.) between the Teensy output pin & the relay control input.
Hope that helps . . .
Mark J Culross
KD5RXT
Is that me?I can assure you that what jmarisma points out is physically confirmed.
Thanks, I will set it up as you described..