I went ahead and fished out my copy of Mountaineering: The Freedom of the Hills, which happens to be the 50th anniversary 8th edition.
The reason there isn't much detail in the list you quote is that it's in the primer chapter. In chapter 17, Glacier travel and crevasse rescue (p.375), you have to go to the Fundamentals of glacier travel (pp.381-382) in the Rope Teams paragraph. There, you find a good explanation of the line you quote:
Rope teams of three climbers each are ideal for travel on glacier where no technical climbing will be encountered. With a rope team of three, two people are available to arrest a rope mate's fall into a crevasse. A minimum party size of two rope teams is recommended so that a team involved in an accident will have backup help. In some instances, a party of four may climb on a single rope; for example, if one of the climbers may not be able to arrest a crevasse fall, or if just one of the party is experienced in crevasse rescue.
On technical glacier terrain - with slopes steeper than 40 degrees or with severe crevassing - belaying may be necessary, making it more efficient to travel in two-person rope teams. In this situation, having a second rope team as rescue backup becomes even more important. While the person who is on the same rope as the fallen climber holds the rope fast, the second team can setup a snow anchor and initiate the rescue (see "Crevasse Rescue Response" later in this chapter).
As mentioned in other answers, if you can't meet the safe minimums for rope teams for glacier travel, it is probably the best idea to change plans. 4 climbers could potentially rope up into two teams or as one team, but in the second case it requires a full-length rope (or double ropes):
Glacier travelers usually put 3 people on a 37-meter (120-foot) rope, and three or four people on a 50- or 60-meter (165- to 200-foot) rope. These configurations space the climbers far enough apart so that as the rope team crosses a typical crevasse, only one person at a time is at risk. Where there are truly humongous crevasses - in the himalaya or the Alaska Range, for example - greater spacing may be necessary.
A team of more than 5 becomes more risky as the distance between members isn't large enough for safe travel. There is also an efficiency component as the larger the rope-team, the worse the rubber-banding will affects travel speed as each member tries to keep the rope taut.