Unsure about parse tree of PCFG
Your tree is correct. However, it's not the only possible one. Your tree came from the derivation
$$
S\Rightarrow Sa\Rightarrow aSaa\Rightarrow aaaa
$$
a different one would be from
$$
S\Rightarrow aSa\Rightarrow aSaa\Rightarrow aaaa
$$