R4 door? That's a pretty big 738!
While it's not recommended to open aft doors (as another poster mentioned) it's not an absolute no.
A number of factors may have come into the F/A decision- given the proximity to land, the depth of the water and the presence of boats in the water immediately after, s/he may have felt it necessary and wiser to get the pax out asap.
From what I can recall, the boarding of rafts out of the OWE and L/R1 doors procedure is designed after a ditching where the aircraft must be kept floating for as long as possible (for example, the Hudson or other fairly open water) Since the time required to 'float' would clearly have been less in this situation, then that may have come into play. Its possible the crew were under the impression the plane was on the reef and pax could 'wade' from there?
Of course the SOP is there for a reason but the manual/manufacturer's textbook cannot cover every possible situation, That's why there's a human sat in that jumpseat and not a Coke machine in the back galley.
Not to mention depending on the crew it may or may not be possible to keep from being trampled by the back half of the plane trying to get out. Of course most pax would not have bothered to read the safety card which clearly shows the aft doors NOT used in a ditch. FA are also taught that at some point you will need to make the call to get out- if it was open or be trampled to death then it may have been the only option at the time.
Sorry for the drift, just wanted to address those points from the FA viewpoint... carry on!