If the population growth (P) of a sample of bacteria, after $x$ days is expressed as a function $P(x)=4 * 4^{x}$. What is the population of bacteria after 3 days?

Step 1 ：Given the function for population growth: \(P(x) = 4 * 4^{x}\)

Step 2 ：We need to find the population after 3 days, so we need to find \(P(3)\)

Step 3 ：\(P(3) = 4 * 4^{3}\)

Step 4 ：\(P(3) = 4 * 64\)

Step 5 ：\(P(3) = 256\)

Step 6 ：\(\boxed{The population of bacteria after 3 days is 256}\)