A lot of times, I've seen eighths in groups of 2, and other times in groups of 4.
Is there any difference in the way you are supposed to play them?
No, not directly. The beaming may affect how you think about the time subdivisions (four groups of two eighth notes vs. two groups of four eighth notes) which might subtly affect your accenting of the overall piece, but it's not really an explicit thing, no.
For comparison, in choral music, you often see all the eighth notes unbeamed, unless they belong to the same syllable of a word.
Here's some more information as well: http://www.ars-nova.com/Theory%20Q&A/Q4.html
As that page points out, the beaming does have some indication of meter, in that you wouldn't typically beam across the middle of a 4/4 measure, for example. (Just to be clear: this is more of a case of meter informing beaming, not beaming affecting performance).