= Solution
Use <conjunction elimination> to obtain both premises needed for a contradiction under a temporary implication assumption. The following <natural deduction> derivation discharges precisely assumption 4:
$$
\begin{array}{rll}
1&Q\land P&\text{premise}\\
2&P&\land E\ 1\\
3&Q&\land E\ 1\\
4&\quad P\to\neg Q&\text{assumption}\\
5&\quad\neg Q&\to E\ 4,2\\
6&\quad\bot&\neg E\ 3,5\\
7&\neg(P\to\neg Q)&\neg I\ 4\text{--}6
\end{array}
$$
Here $\to E$ is the <implication elimination rule> and $\neg I$ is <negation introduction>. The original conjunction remains the only undischarged premise.
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