Unique {0} type 1
The cells {3} form a Unique {0} with the values
{1} and {2}. There are exactly two ways of placing
the values {1} and {2} in the cells of the
Unique {0}, forming two possible configurations.
In both configurations, each row, column or block touched by the Unique
{0} contains each of the two values {1} and {2}
exactly once. As a result, if one of these two configurations were part of the
solution, it could then be replaced by the other one to get a second valid
solution.
Because a valid sudoku cannot have more than one solution, none of the two
configurations of the Unique {0} can be valid. The only possible way to avoid these
two configurations is if the cell {4} does not contain the values
{1} and {2}.