Teensy 4.0 Breakout Kit

Thanks so much for the replies regarding header and pin sets. I was not aware of the offering from Adafruit that matches the Teensy so well. Was not aware of Protosupplies at all, that looks like a good resource and the fact their female headers are snappable makes them very attractive.
I get them from Adafruit...

Low Profile Headers: Adafruit are good.

One in p#423 are breakable Male and Female. The AdaFruit Females are solid and lose a pin when cutting to length ... with more effort to cut and clean the overage.

The breakable ones came up in an @loglow thread and cost more but are nicer. Ken at ProtoSupplies got some in stock when I asked him about them.

Also those headers sit even lower as the short end of the MALE can fully connect in the Female where the longer end won't go fully in so it sits higher with pin exposed between headers.
The Adafruit ItsyBitsy headers are great. Though connecting VUSB might be an issue.

However, I find the Sparkfun stacking headers for the Teensy 3.2/4.0 to be less useful. The ItsyBitsy headers are 2 14-pin headers and one 5-pin header. The Sparkfun headers are 2 13-pin headers and one 7-pin header. For breadboarding, etc. I find it easier to use a stacking header for the back 5 pins (or solder in the stacking header, and just clip off the pins). In addition, the 14-pin headers work better on the Audio Shields (which don't have the back row of pins). I presume it would also work better for the Octows2811 shield, but I don't use that.