Is the change in space permanent, i.e, is it possible to upload >4GB images, get 4.5GB extra space, then delete the images and fill the space with other data at

Is the change in space permanent, i.e, is it possible to upload >4GB images, get 4.5GB extra space, then delete the images and fill the space with other data at a later time, or does the space shrink once the media is deleted?