My attempt at a more friendly description of S4 (based on my very limited understanding of it - please correct me if I'm wrong):

When processing large amounts of streaming data, you have to process the data as fast as it comes in or else you can't keep up. You probably wonder: "Why not simply delay the processing of new data while the old data is being processed?". The problem lies in the fact we're dealing with a stream which always brings in new data. Eventually, the virtual line up of delayed data will occupy all available memory.

A solution to this is to dispatch the data between multiple computers which can each independently process the data they receive and then send back the result of their processing to a central computer whose job is to put back the results together. Can't keep up with the stream? Simply add a new computer! That's more or less what S3 does.