When formatting a DVD-R or DVD-RW disc, a portion of disc space must be reserved for storing HFS and ISO data structures, and for catalog information. Other types of disks also dedicate space to formatting. The amount of space used may seem more noticeable due to the larger capacity of DVD media.
You should have just under 4.3 GB of space available after formatting your DVD disc. You can verify capacity and available disc space by selecting the disc's icon and choosing Show Info from the File menu.
For more information, see technical document 8647, "
Macintosh: File System Specifications and Terms".