4．已知i→\overrightarrow{i}i和j→\overrightarrow{j}j是两个正交单位向量，a→=2i→+3j→\overrightarrow{a}=2\overrightarrow{i}+3\overrightarrow{j}a=2i+3j，b→=i→+kj→\overrightarrow{b}=\overrightarrow{i}+k\overrightarrow{j}b=i+kj且∣a→−b→∣=2\vert \overrightarrow{a}-\overrightarrow{b}\vert =\sqrt{2}∣a−b∣=2，则k=(k=(k=( )))