bluetooth versions have different max. speeds for data transfer... 4.0 version will offer the highest speed...
all 4.0 versions will recognize older versions, BUT...
will NOT always work on lower speeds or older protocols, because of specific appliance needs...
a 4.0 dongle will work with almost any bluetooth appliance
BUT...
a 2.0 dongle will NOT work with some 4.0 appliances
e.g.: 4.0 dongle will work with a 2.0 headset and 4.0 headset and 4.0 xbox one s controller
BUT...
2.0 dongle will work with a 2.0 headset and 4.0 headset BUT NOT with a 4.0 xbox one s controller
it depends on the appliance needs VS dongle capabilities...