Here is a guide I made up for a UD3R & a Q9550 but some of the stuff will apply to you... now mind you the UD3R P45 and UP3P boards are thee best quad 775lga socket mobos for overclocking your Q9550... Other board will help you reach 4Ghz but this board gives you fsb headroom to play with... I loved my retired Q9550 setup..
Make sure you have the latest bios first of all.
Now, if you have all dimm slots occupied it will be alot harder running high fsb for something like 4Ghz... my 4Ghz is done by 500fsb by an 8 multi. I am running a 2x2 config & DDR2 1000 so I don't have to worry about hitting a memory wall when trying to overclock; thus 500fsb is my max without overclocking the ram. My Q9550 is almost near the Intel vcore max. In bios it's at 1.375 but with vdroop it is a steady 1.34v and stable.
First I would drop your multi to 6 and run Memtest86 vX.XX to see if your ram is stable at 1:1 ratio. You'll need to burn Memtest to a disc and boot into it. So say if you have DDR 900 then 450fsb would be 1:1 and DDR 800 would be 400fsb at 1:1 and so on. This way you test to see if your memory isn't the problem. After about 3 100% passes in Memtest86 (you can do this longer but i'd do 3 at minimum) boot into windows at the same low multi of 6 and 1:1 ratio of your ram. Mine was 6mulit x 500fsb = 3Ghz in windows. This way I knew there wasn't so much pressure on the CPU and RAM there-by isolating the motherboards subsystem. This is where you test the NB. One of the best apps to test the MB subsystem (NB circuitry) is Prim95 Large FFTs.
So, when you start getting blue screens and or not booting into your OS that's when you slightly raise those settings. PLL & MCH (MCH is your NB) would be a good place to start. Then, if you keep failing your boot or blue screen in P95, raise your Vcore: but keep that Termination at a difference of .0500v
With that, you should keep testing till you reach your OC goal. Be careful not to overheat your chip beyond 70*c on the cores. 70*c is conservative but good advice.
You may also want to test your system stability with LinX - maybe 20 or 30 passes at the most memory you're allowed by the app. This will heat your system up tremendously so keep an eye on your core temps. I personally use Hardware Monitor which monitors your core temps and Northbridge temps as well as giving your voltage readings, fan speeds, etc..
If you have any more questions feel free to pm me.
Have Fun and Good Luck!
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