[tex]4 + \sqrt{ - 16} \\ = 4 + \sqrt{ - 2 \times 2 \times 2 \times 2} \\ = 4 + 2\sqrt{ - 4} \\ = 2(2 + \sqrt{ - 4} )[/tex]
Hope you could get an idea from here.
Doubt clarification - use comment section.
[tex]\\ \sf\longmapsto 4+√-16[/tex]
[tex]\\ \sf\longmapsto 4+√16i[/tex]
[tex]\\ \sf\longmapsto 4+4i[/tex]
[tex]\\ \sf\longmapsto 4(1+i)[/tex]
