Which one of the combinations of the following statements is true about antibody structure?

1. Limited proteolysis of rabbit IgG with the enzyme pepsin generates two antigen-binding regions (Fab) and an Fc fragment.
2. Limited proteolysis of rabbit IgG with the enzyme papain generates a single bivalent antigen-binding region F(ab')$_2$ and peptide fragments.
3. The Fc fragment of IgG can self-associate and crystallize into a lattice.
4. The F(ab')$_2$ fragment of IgG is composed of both light and heavy chains.
1. P and Q only
2. P and R only
3. R and S only
4. Q and S only