Answer
The bacteria population after one hour is $~5443$
Work Step by Step
We can find the population $P(1)$ after one hour:
$P(1) = 4000+\int_{0}^{1}1000\cdot 2^t~dt$
$P(1) = 4000+1000\cdot (\frac{2^t}{ln~2})\vert_{0}^{1}$
$P(1) = 4000+1000 \cdot (\frac{2^1}{ln~2}-\frac{2^0}{ln~2})$
$P(1) = 4000+1000 \cdot (\frac{2}{ln~2}-\frac{1}{ln~2})$
$P(1) = 4000+1000 \cdot (\frac{1}{ln~2})$
$P(1) = 5443$
The bacteria population after one hour is $~5443$