It's a Spectator, not a Beholder. Spectators are CR 3. A level 4 party should be able to wipe the floor with one, especially since BG3's spectator does not have the Spell Reflecton ability. The difficulty with this fight is all the drow it unpetrifies.
The strategy for this fight is to deal some damage to the drow so that they help attack the spectator...or make use of of the powerful level 4 abilities/items that you should have. A successful fighter's Menacing Attack basically wins the fight.
