Hi,
In brief - the "map/reduce" nature of S4 reflects in the fact that data processing is based on key aggregation (multiple events ending up at the same PE, a'la mapreduce)
while the "Actor" nature of S4 reflects in the fact that PE's are stateful and can perform arbitrary computation and/or spawn new events (unlike mapreduce and along the lines of traditional actor model).
best,
.Alek