The webpage www.johnstone-music.com has a huge selection of fine transcriptions for all sizes of cello ensemble including almost 140 for cello quartet. Clicking on the highlighted link will take you directly to the page of cello quartets on that wonderful website. Be aware that the number of separate cello voices does not limit the maximum size of the group. For example, while these pieces of music will normally have four different individual voices (and will require a minimum of four players), they can in fact be played by larger groups, simply by doubling the voices. Watch out also for the difficulty level: David Johnstone is a virtuoso cellist who likes to exploit the full potential of the cello, so for students it may be well to keep to the lower voices and/or to choose easier (more pedagogical) arrangements of which there are also very many on his site.
Here is another webpage with some very nice – and simple, easier, more pedagogical – arrangements of Bach keyboard works for cello ensemble (usually four cellos):