There are 439,400 valid 4-letter asset names. I don’t know how many of these that have been registered yet, but out of a total of 30k assets it’s likely that less than 10k are four letter names.
An asset name can start with any letter except A, and that’s why there are 25 x 26 x 26 x 26 = 439,400 four-letter combinations.
It’s unlikely all combinations will be registered anytime soon. With 0.5 XCP per registration and the price of XCP at around $0.90, it would seemingly only cost around $200,000 to register all. However, to buy enough XCP you would press up the price significantly.
All good combinations is another story. Say you’re only interested in names of format consonant-vowel-double-consonant, e.g. JIFF or WALL. Then the number of combinations is just 21 x 5 x 21 = 2205.
For the record, there are 11.4 mln valid five letter names, 297 mln six letter ones, 7.7 bln seven letter ones, and so on.
Conclusion is that the namespace is practically endless, even for short names, but if you want to register a good name you better hurry.