Expandotetration refers to the function \(a \{\{4\}\} b = \{a,b,4,2\} = \underbrace{a \{\{3\}\} a \{\{3\}\} \ldots \{\{3\}\} a \{\{3\}\} a}_{b ext{ a's}}\), using BEAF. In the fast-growing hierarchy, \(f_{\omega+4}(n)\) corresponds to expandotetrational growth rate.