First you must find the Pressure Height, and find the standard temperature there, for comparison with the actual temperature.
Sea Level at 975 hPa = 1061 ft Pressure Height
Standard Temperature at 1061 feet = 12.9°C
ISA Deviation = +4.0 - 12.9 = ISA - 8.9°C
Density Height Correction = - 8.9 X 120.9 = - 1076 feet
Density Height = 1061 -1076 = - 15 feet
In your example the low temperature has very nearly compensated for the low pressure, in giving a Density Height very near to the 'original' altitude.
(Note that the effect of QNH variation in calculating Pressure Height is logarithmic, not linear, and in this case was 27.74 feet per hPa).
Regards,
Old Smokey