$\text{P}$ and $\text{Q}$ are two square matrices of the same order. Which of the following statement(s) is/are correct?

1. If $\text{P}$ and $\text{Q}$ are invertible, then $\text{[PQ]}^{-1} = \text{Q}^{-1} \text{P}^{-1}.$
2. If $\text{P}$ and $\text{Q}$ are invertible, then $\text{[QP]}^{-1} = \text{P}^{-1} \text{Q}^{-1}.$
3. If $\text{P}$ and $\text{Q}$ are invertible, then $\text{[PQ]}^{-1} = \text{P}^{-1} \text{Q}^{-1}.$
4. If $\text{P}$ and $\text{Q}$ are not invertible, then $\text{[PQ]}^{-1} = \text{Q}^{-1} \text{P}^{-1}.$