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Rather than buying buckets or carboys, I would like to use 5 gallon PET water bottles that I can get free from work (I verified they are PET with the "1" on the bottom). However all my extract recipes are for 5 gallon batches, which would not leave any room for fermentation.

Is there any harm in reducing the volume to about 4 gallons for fermentation in these containers? I like higher gravity beers, for what it's worth, but I don't know if there's an upper limit. My next batch is going to be a Russian Imperial Stout (OG: 1.082).

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Fermenting in a the free 5 gallon bottles you can get will work fine - after all that's pretty much what Better Bottle PET Carboys are. Compared to the 2-2.5 gallons of headspace you get in a bucket fermentor, the 1 gallon that you're leaving isn't much, especially for a high gravity beer like a RIS, so be sure to use a blowoff tube rather than an airlock.

It's probably best to also scale the recipe size down from 5 gallons to 4 gallons, unless you really like very big beers! (1.082 RIS 5 gallon recipe brewed at 4 gallons will come out at 1.102.)

blowoff tube

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