Let the hundreds digit be $x$, the tens digit be 0, and the ones digit be $y$. The original number is: $100x + 10y$

The number formed by interchanging the first and last digits is: $100y + 10x$

The given information is: $100y + 10x - (100x + 10y) = 198$

Simplifying: $90(y - x) = 198$

Dividing both sides by 90: $y - x = 2$

Also, it is given that the sum of the digits is 4: $x + y = 4$

Now, solving these two equations: $x + y = 4$ $y - x = 2$ Adding both equations: $(x + y) + (y - x) = 4 + 2$ $2y = 6 \rightarrow y = 3$

Substituting $y = 3$ into $x + y = 4$: $x + 3 = 4 \rightarrow x = 1$

Therefore, the difference between the first and last digits is: $y - x = 3 - 1 = 2$