Server Keeps Crashing

Here is the crash report. I don't know why but its the 4th time today I have 4GB of Ram and only had 4 people on at the time.

Code:

---- Minecraft Crash Report ----
// Oh - I know what I did wrong!
 
Time: 10/6/13 4:35 PM
Description: Exception in server tick loop
 
java.lang.OutOfMemoryError: unable to create new native thread
    at java.lang.Thread.start0(Native Method)
    at java.lang.Thread.start(Unknown Source)
    at net.minecraft.server.v1_6_R3.NetworkManager.d(NetworkManager.java:315)
    at net.minecraft.server.v1_6_R3.PendingConnection.a(PendingConnection.java:179)
    at net.minecraft.server.v1_6_R3.Packet254GetInfo.handle(SourceFile:72)
    at net.minecraft.server.v1_6_R3.NetworkManager.b(NetworkManager.java:296)
    at net.minecraft.server.v1_6_R3.PendingConnection.d(PendingConnection.java:49)
    at net.minecraft.server.v1_6_R3.DedicatedServerConnectionThread.a(DedicatedServerConnectionThread.java:41)
    at net.minecraft.server.v1_6_R3.DedicatedServerConnection.b(SourceFile:29)
    at net.minecraft.server.v1_6_R3.MinecraftServer.t(MinecraftServer.java:592)
    at net.minecraft.server.v1_6_R3.DedicatedServer.t(DedicatedServer.java:227)
    at net.minecraft.server.v1_6_R3.MinecraftServer.s(MinecraftServer.java:488)
    at net.minecraft.server.v1_6_R3.MinecraftServer.run(MinecraftServer.java:421)
    at net.minecraft.server.v1_6_R3.ThreadServerApplication.run(SourceFile:583)

 

Post your server.log please.