The growth in value of 3-letter .coms are partly responsible for elevating the value of 4-letter .coms. In 2001, you could get 3-letter .coms in the $500 to $2,500 range. Now the range begins around $5K for less-popular letter combinations.
Also, all 4-letter .coms have been registered, although the less-popular letter combinations often go through cycles of dropping and re-registration.
The better 4-letter combinations often have a number of uses and match different things, and often take on new meanings as time goes on. The fact that all the 4-letter .com real estate is gone raises the price, as demand grows for short .coms in general.
Many 4-letter .coms get significant traffic and earn good PPC revenue, however, it's hard to predict which ones will do good, unless there are obvious uses (as indicated by a Google search).
I also think that, with growing Internet users around the world, more people and businesses will want affordable short .coms, and 3-letters may be out of reach for most - so that leaves 4-letter .coms as an option, unless they want to go for something a bit longer with digits or multiple words.
Although there are almost 457,000 LLLL.com combinations, that is still a relatively small number in the total domain world. The lesser-popular letter combinations are currently limited in value (those with J, K, Q, U, V, X, Y, Z), and those will gain value much slower than popular combinations. Also, some of those "less-popular" letters may be more popular in other countries.