What causes speaker impedance fluctuated ? For instance for BW DM-303 it marks nominal ohms = 8 and minimum = 4.3 What causes ohm changed ? Music ?

For speaker set-up in my Yamaha receiver it marks :

4 ohms or higher for each speaker if set A or B is used and
8 ohms or higher for each speaker if sets A and B are used

Currently I am using 2 sets of front speaker A and B at the
same time. With A and B, I am connecting to 2 pairs of 8 ohm speakers.
However, in speaker specifications it indicates minimum ohm = 4.3
If it's the case when speaker impedance goes to minimum (4.3) will
it fry or toast (burn) the receiver ? Thanks

Receiver : Yamaha RXV-420
Speakers : BW DM-303