Assuming four variants of the J-20 actually exists, I assume it's:
J-20 - original J-20s produced with AL-31s
J-20A - WS-10C equipped with upgraded avionics (likely what consists of most of the current J-20 fleet and has been produced the last few years)
J-20B - new single-seat with dark coat and hump
J-20S - new two-seater version with dark coat and hump
J-20 - original J-20s produced with AL-31s
J-20A - WS-10C equipped with upgraded avionics (likely what consists of most of the current J-20 fleet and has been produced the last few years)
J-20B - new single-seat with dark coat and hump
J-20S - new two-seater version with dark coat and hump