This means that these classes cannot be kitted for a PC, correct? NPC only kits for these classes can be made, can't they? (Good to know before I start trying to tinker about) Err, no. These four really are hardcoded and unkittable. With a custom NPC though, you can grant extra abilities and such with a script that grants extra abilities at level-up.
I believe Sorcerers and Monks can be kitted, but you can't select the kits when creating your character, you have need to edit their kit in-game with AddKit() or by using Shadowkeeper.
The values for Sorcerer and Monk are 19 and 20, respectively, from class.ids.
I did a bit of testing with this yesterday and discovered that for trueclass, multiclass, and dual classes whose ORIGINAL class is Mage, the spells are added properly (upon gaining a level in any class). Dual classes who start as something other than mage seem to never gain the extra spells. Also, going from specialist to trueclass will reduce the number of spells to the proper amount on levelling.