Make default number of parallel fetches slightly smaller since it doesn't seem...

Make default number of parallel fetches slightly smaller since it doesn't seem to hurt performance much and it will cause slightly less GC.

core/src/main/scala/spark/storage/BlockManager.scala

@@ -631,7 +631,7 @@ class BlockManager(val master: BlockManagerMaster, val serializer: Serializer, m
 object BlockManager {
 
   def getNumParallelFetchesFromSystemProperties(): Int = {
-    System.getProperty("spark.blockManager.parallelFetches", "8").toInt
+    System.getProperty("spark.blockManager.parallelFetches", "4").toInt
   }
 
   def getMaxMemoryFromSystemProperties(): Long = {