There are 16 possible 4 bp restriction sites; not all possible sites correspond to a known endonuclease activity. There are also alternative enzymes (isoschizomers) that  recognize the same site, but cleave it in a different manner. For example, MboI and DpnI both recognize GATC, but the first cuts it 5' to the first base [/GATC] and the second cuts it between the two middle bases [GA/TC]. As well, some enzymes recognize the same site but cleave it differently according to its state of methylation. For example, MboI and Sau3AI both recognize GATC, but the former will not cleave the site if the A base is methylated. Methylation is used by some bacteria to protect its own DNA from self-digestion. [Click here for further information on Restriction Endonuclease sites].