The straight-line method of depreciation is given by:
\[
\text{Depreciation per year} = \frac{\text{Cost} - \text{Salvage Value}}{\text{Life}} = \frac{160000 - 10000}{5} = 30000 \, \text{₹}.
\]
Thus, the book value at the end of the 4\(^\text{th}\) year using the straight-line method is:
\[
\text{Book Value} = 160000 - 4 \times 30000 = 160000 - 120000 = 40000 \, \text{₹}.
\]
For the sum of years digit method, the sum of the years is:
\[
S = 1 + 2 + 3 + 4 + 5 = 15.
\]
The depreciation fraction for the 4\(^\text{th}\) year is:
\[
\frac{4}{15}.
\]
Thus, the depreciation in the 4\(^\text{th}\) year is:
\[
\text{Depreciation} = \frac{4}{15} \times (160000 - 10000) = \frac{4}{15} \times 150000 = 40000 \, \text{₹}.
\]
The book value at the end of the 4\(^\text{th}\) year using the sum of years method is:
\[
\text{Book Value} = 160000 - 4 \times 40000 = 160000 - 160000 = 0 \, \text{₹}.
\]
The difference between the book values is:
\[
40000 - 0 = 20000 \, \text{₹}.
\]
Thus, the difference in book values is \( \boxed{20000} \, \text{₹} \).