Alpha-glucosidase I and II have high activity towards malto-oligosaccharides and starch, while form III and IV have high activity towards malto-oligosaccharides but low activity toward starch. Hydrolysis of terminal, non-reducing (1->4)-linked alpha-D-glucose residues with release of alpha-D-glucose. Four different forms (I-IV) may be produced by post-translational modification. Belongs to the glycosyl hydrolase 31 family.