General Bukkit server memory help

Me and my friend have a bukkit server running with 40 slots, but are having some problems with freezes when the memory usage gets to the limit we have set for the server.

When we start the server and it´s full it only uses around 1 to 2 gigs of ram, but eventually it will reach the 4 gigs and most of the time freeze after a random timeframe (with rollback).

We notice it keeps growing when players logg in/out, but it does not free up any memory.

-For the craftbukkit version, we keep testing the latest dev builds for 1.4.5.
-The server runs on Windows 7 x64

Code:

@echo off
java -Xincgc -Xmx4000M -jar craftbukkit.jar nogui
pause

Code:

java version "1.7.0_07"
Java(TM) SE Runtime Environment (build 1.7.0_07-b11)
Java HotSpot(TM) 64-Bit Server VM (build 23.3-b01, mixed mode)

We run this plugins:

- Worldguard + Worldedit
- Vault
- Factions
- Multiverse with a creative world
- Essentials
- Dynmap
- PEX


I know bukkit 1.4.5 is still a dev version, but could this be a bukkit or plugin problem?
Is this memory usage normal? do we need more memory?

Is there any way to see what plugin is using how much memory maybe?

 

How often do you restart? bukkit servers usually don't perform well after 24 hours. If you don't have a automatic daily restart, is should fix your problem.

 

I say reboot 2-4 times a day (2 = 1 per 12 hours, 4 = 1 per 6 hours). Seems to keep my usage pretty low.