MichaelMeissner
Senior Member+
You can daisy chain them, connect board 1 to board 2, connect board 2 to board 3, and connect board 3 to board 4. That way you only use 2 grounds on each Teensy.
Generally though what I would do is either use traditional breadboards or prototype boards that have power rails on the outside. On each Teensy, you connect the ground wire to the black power rail. Then you can use this power rail to connect to the next Teensy or all 4. I always connect 3.3v to the red power rail, but you would not want to interconnect the power rails between the Teensys. If you were attempting to power each of the Teensys with one power supply, then you would connect VIN instead of 3.3v to a power rail, and then interconnect the Teensies.