Question

This is a worked example of an expression containing exponents from my IB workbook:
a) (2^x + 3)(2^x + 1)
= 2^x * 2^x + 2^x + 3 * 2^x + 3
= 2^2x + 4 * 2^x + 3
= 4^x + 2^2+x + 3
I understand everything except for how they got the 4 in step 2. Can someone please help me understand this?