Are 4 blank passport pages enough to go to Spain and France?
 

I have four blank one-sided passport pages (not four 2-sided pages). We're going to Spain and France in October.. Do I need to buy more passport pages? I still have two years left on the passport..

No you will have at most only two stamps - one on arrival in the Schengen Zone and one for departing the Schengen zone - none between Spain and France and stamps are rather small.

Can you buy extra passport pages? Just curious. Once when returning to the States my passport was full from Asian travels with big visas, etc and Customs simply added a few pages to my passport at no fee. But that was before 9-11.

Assuming you don't need any visas which can take full pages then, yes, it's enough.

In the US, you can send your passport to the state department to have more pages added for about $80.

The powers that be have no problem stamping over a previous stamp,
if they stamp at all.

This is not an issue.

The only reliable place anymore to get a stamp from some country is the U.S.

My wife likes to 'collect' country stamps, so she pleads for the various countries to stamp her passport.

Me, I could care less.