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Will any 4 bolt chainring fit any 4 bolt spider?
No. It depends on the diameter of the circle that goes through the centre of all of the bolts, the BCD (or PCD). For example the granny ring of you have one is normally much smaller. If you take a look at wiggle you will see lots of different rings with different BCDs
http://sheldonbrown.com/cribsheet-bcd.html
Sorry, I missed out I meant middle 4 bolt chainrings, my brothers got a GXP spider, putting a 32 tooth middle ring on, i've got an old 32 tooth deore ring and it was way too small, about 2mm out. How do I know what size to buy?
Use the crib sheet on the link I gave you above. It will allow you to measure the distance between two of the bolts and then convert that into the BCD that you need.
That's belting mate! Problem solved!
Cheers!