Wordbiker

^What he said.

I'd also add that I have seen quite a few 4-bolt MTB cranksets come into the shop with bent rings. Although I agree with DMF's theories, MTB drivetrains are more often shifted under duress than road, and should be stronger. The issue is a simple matter of distance between bolt centers. The chainring is spanning a greater distance with the 4-bolt design and can be distorted by the side loads of shifting under power. Sure, the chainring could be reinforced, but then you'd regain the weight savings over a 5-bolt design.

None of these would cause me to remove a 4-bolt crank from a bike just to replace it with a 5-bolt, but if I were choosing new parts, it would be worth considering.