FRDM4236 (Trainder4EDU) has created an expansion board for Teensy 4.0, similar to Dean’s, but providing all the extra signals & features in a Teensy 3.6-list form factor.
The Trainer4Edu website has detailed information on the board as well as build instructions for you to build your own.
Dean Blackketter created an add-on board which makes Teensy 4.0’s bottom-side pads available as a second row of normal pins!
It attaches underneath with castellated holes to solder to Teensy 4.0’s bottom-side pads.
If you’ve wanted a way to plug Teensy 4.0 into a sturdy through-hole socket and have access to all the signals, this looks like a really nice way to gain easy access to those bottom-side pads.
Dean shared this PCB on OSH Park and the Kicad files on Github. Much more detail is available on Dean’s forum post and conversation which follows.