Not terribly surprising there seems to be a rush on getting Teensy 4.0's.
Yesterday, I noticed that Adafruit had them, and I debated ordering immediately, but I waited until the evening to order mine. Yesterday, they didn't mention the amount in stock (they don't list the stock level until the stock level drops below 100). As I'm posting (Thursday August 8th, 7:45pm EST), they are now down to 64 Teensy 4.0's. By 8:00pm, it was down to 57.
Right now, Sparkfun has 163 T4's. I didn't think to check the stock level yesterday. Sparkfun does limit orders to 5/person.
When I checked it, PJRC still had stock, but now they list it as low availability.
Digikey don't yet show it. I've noticed it is usually a few weeks after Adafruit or Sparkfun announces something that it shows up in stock (and both Sparkfun and Adafruit provides different Teensys that Digikey resells). I don't normally use Mouser but from the other Teensys they have, it looks like Digikey where they buy them from Adafruit or Sparkfun, so I imagine there will be some time before they have it.
I haven't seen it on Amazon or Ebay yet. But like with Digikey and Mouser, I would expect them to get the primary supply from Adafruit and/or Sparkfun.
I also checked Microcenter, and they haven't seen it yet, but Microcenter has not really been reliable in stocking Teensys.
I tend to order from Adafruit, since as a NY company, it gets to me quicker, particularly if I order it after 8:00pm on Wednesdays and use the 10% off code in Ask an Engineer, it almost always gets to me on Friday morning (I live in Massachusetts). If I don't use the code, it usually takes another day or two to process the order.