Thanks for your replies!
And lakersfan, thank you.
Back in January, I found one spark plug which gave quite a lot of performance gain.....but it costed US$128, and after few days, ruined one of my spark plug wires, and so i took it out......and I can't remember which one it was.
But I"ll definatley get the +4. How much performance gain do you get out of it? Very noticeable, or just a small difference?