Beryllium and aluminium exhibit many properties which are similar. But the two elements differ in:

Beryllium and Aluminium share a diagonal relationship and many similarities. However:Beryllium is in the second period and has a maximum coordination number (covalency) of $4$ due to the absence of $d$ -orbitals.Aluminium is in the third period and can expand its covalency beyond $4$ using $d$ -orbitals (up to $6$).
Both form amphoteric oxides and covalent halides. Beryllium forms polymeric hydrides, while aluminium forms dimeric covalent hydrides.