Question about Bluetooth Version 5.0 and Version 4.1 hardware backwards compatibility
Bluetooth 5 is backward compatible with Bluetooth 4 until you use any of the new Bluetooth 5 features. If, for example, you tried to use the 2MBPS PHY with a Bluetooth 4 device, it would not work. If your Bluetooth 5 device uses the 1 MBPS PHY, however, it is able to communicate with a Bluetooth 4 device.

Because in Bluetooth 5's protocol stack, at a bottom layer which is a PHY Layer or Physical Layer, totally three variants have been added in which two variants are new and these two are for longer range and double the speed of Bluetooth 4 as mentioned above. 

Name of those variants are


LE 1M (1MBPS PHY)


LE 1M is the PHY used in Bluetooth 4. It uses Gaussian
Frequency Shift Keying and has a symbol rate of 1 mega
symbol per second (Ms/s). It continues to be available for
use in Bluetooth 5 and its support is mandatory.


LE 2M Double the Speed (2MBPS PHY)


The new LE 2M PHY allows the physical layer to operate
at 2 Ms/s and thus enables higher data rates than LE 1M
and Bluetooth 4.


The LE Coded PHY


The LE Coded PHY allows the range to be quadrupled
(approximately) compared to Bluetooth 4, and this has
been accomplished without increasing the transmission
power required.