LG - I agree regarding concerns about search bubbles and personalization. Here's a link to the classic TED talk video on the subject...
Eli Pariser: Beware online "filter bubbles"
That said, I don't think that Google is testing here whether they should eventually return only four results. I'm guessing that they use limited tests like this to help calibrate very broad changes in direction only. I'm not enough of a statistician to make an arguement for this. It just feels to me that that's what they'd be doing.
It's consistent for me in how one should structure site navigation... that in a site with fixed nav, you get increased focus as you drill down, with of course also links to appropriate related content. Search, of course, is different.
How much Google search results might come to resemble this aspect of site nav structure is complete speculation, but it makes sense to me that Google would investigate changes of direction within a series of searches. That doesn't automatically reduce results to four choices, but for a test, four choices might produce some interesting data. Ultimately, I don't think that Google is about to throw away user engagement with its site (if, in fact, it's there) by shrinking its serps.
Regarding broader questions of personalization, questions of time-frame, short and long term choices, and avoiding search bubbles are dimensions that Google would need to investigate.