Bats and pterosaurs have skin attached to their forelimbs that aid them in flying and when they are on land, they use all four of their limbs to walk in a quadrupedal posture. Can these organisms be considered as true quadrupeds? Is there a different term used for this type of locomotion?
Quadrupedal means walking on 4 limbs, so yes bats can be considered quadrupeds. The scientific name for how an animal walks is gait. However, keep in mind that the same animal can have different gaits, think of a horse that can walk, gallop, etc. It seems that not all bats walk the same way, this here might be of interest, where they tell about a paper published in Nature in which they find a very different gait in Vampire bats, compared to 'normal' bats. In this paper they say that normal bats: "...shuffle awkwardly from a sprawled position." So I don't think they have a real name for a 'normal' bat gait.
For flying pterosaurs, it is hard to tell, we've never seen one walk.