I have a MBP 17" (late 2011) and would like to use the new Magic input devices (Keyboard, Mouse 2, and Trackpad 2) with it which seem to rely on Bluetooth 4. How can I connect them to my MBP?
You could just use it wired, meaning connect the lightning cable that is usually used to charge the pad to connect it to your MacBook, it works.
Some people tried using it with older Macs and it seems to work with Bluetooth 2.0 as well, but i doubt the battery will last as long as with Bluetooth 4.0.
The chip in the device itself is only capable of Bluetooth 3, but Apple never sold Bluetooth 3, they went from 2.0+EDR directly to 4.
You will however need El Capitan, without that it won't recognize the device.
I can offer that the Magic Trackpad 2 works with my mid-2011 iMac, although as was suggested, the battery life on it is reduced from the promised month down to a bit less than two weeks per full charge.
Also, I am encountering a problem which may or may not be related to this unsupported configuration. Occasionally, I'll tap on a window in the background, and El Capitan will bring the window to the foreground, but the previous application's window will retain focus, causing keyboard input to be directed seemingly to the wrong application.
I did not have this particular problem with the original Magic Trackpad.
I also didn't see it if I use the Magic Trackpad 2 plugged in instead of wireless, although I didn't fully test that configuration.
Subsequent testing with my Magic Trackpad 2 plugged in continues to show the same problem, so it's not Bluetooth-related.