the entire point was that it's not mapreduce, it's a stream processing system. try http://wiki.s4.io/Manual/S4Overview#toc1 for a "what is S4" intro.

The architecture resembles the Actors model, providing semantics of encapsulation and location transparency, thus allowing applications to be massively concurrent while exposing a simple programming interface to application developers.

and

S4 is a general-purpose, distributed, scalable, partially fault-tolerant, pluggable platform that allows programmers to easily develop applications for processing continuous unbounded streams of data.

go stream processing. also see Flume for related recent works.