I’ve had a chance to read about the Pi 4 and think about it a bit, although I still haven’t been able to get my hands on one. It’s still a four-core ARM processor, but now at 1.5 GHz. The gigabit Ethernet might actually run at rated speed, and two of the four USB ports are USB 3.0. The 40-pin GPIO is the same, and no word on making the pins 5V-tolerant, an upgrade that I’d really like to see. With one GB of RAM, it’s still $35; you can get up to four GB of RAM for $55. Brown’s Rule of Computers says, “Never buy less memory,” so we should probably think of the Pi 4 as a $55 device, not $35.
These changes make the Pi 4 a huge improvement for embedded systems, but what about teaching?
Most of the things students do with the Raspberry Pi do not tax either the processor or the memory, and so, in most cases, students will not notice any operational difference between the Raspberry Pi 3 and the Raspberry Pi 4. However, the Raspberry Pi 4 needs a larger power supply and uses different connectors for power supply and monitors. That will make mixing Model 3 and Model 4 in the same environment difficult.
My advice: if you are starting from scratch, start with the Model 4 and get as much memory as you can afford. If you are currently using Model 3, stick with it, at least for the next year.
One other consideration… You might want to replace Windows computers with the Raspberry Pi. You can run Chromium OS, the open-source version of Google’s Chrome OS, reuse monitors (with an appropriate adapter) and keyboards, and maybe not buy a bunch of Chromebooks. I haven’t tried this, so you should do some experimentation before buying a classroom full, but the Pi 4 with 4GB of memory ought to really shine in that application. A couple of caveats… You probably don’t want to use the same equipment for digital making and Chromebook replacement, and the Chromium OS depends on volunteer maintainers. I downloaded a copy from here: https://github.com/FydeOS/chromium_os_for_raspberry_pi/releases and it appears to work as expected.