Answer
First four terms: $4, 0.9, 0.09, 0.009$
Sum: $S = 5$
Work Step by Step
If we ignore the first term, we find that the next term is the previous term multiplied by a factor of $0.1$. Thus, the first four terms are $4, 0.9, 0.09, 0.009$
If we exclude the first term, we find that it's a geometric sequence with the first term $a_0=0.9$ and $r=0.1$, which satisfies $r<1$.
$S_\infty = \frac{a_0}{1-r} = \frac{0.9}{0.9} = 1$
To find the total sum, we need to add the first term and $S_\infty$ together:
$$S = 4 + S_\infty = 4 + 1 = 5$$
