Mmm... Candies

Let $x$ denote the number of $4$ -piece packs and $y$ the number of $5$ -piece packs. The equations to solve are $\begin{array}{rl} x + y &= 19\\ 4x + 5y &= 17\ell \end{array}$ where $\ell$ is the positive integer. In $\bmod\, 17$ , $\begin{array}{rl} x + y &\equiv 2\\ 4x + 5y &\equiv 0 \end{array}$ So $y \equiv -8 \equiv \boxed{9\text{ five piece candy packs}}$ . So we have $10$ four-piece and $9$ five-piece packs as the only solution for the problem since $17$ is prime.

Let $x$ be the number of 5-piece packs. Then, $19 - x$ represents the number of 4-piece packs. The total number of pieces of candy is equal to $5x + 4(19 - x) = 76 + x.$ The smallest multiple of 17 greater than 76 is $17 \times 5 = 85,$ so we conclude $x = 85 - 76 = \boxed{9}.$ The second smallest multiple of 17 greater than 76 is $17 \times 6 = 102,$ but $102 - 76 = 26 > 19,$ so this scenario is not possible.