I have a similar hierarchy to griffen, though I don't think I could trace it back to any sort of dictionary or other hierarchical source. To me:

Couple: two, maybe more "if you're lucky" - "I have just a couple of those left" would be my typical "may or may not be two, but the chances of it being enough in quantity to imply 'few' or 'several' are very low" usage, but usually I use it to imply "two."
Several: Three or more.
Few: Four or more.