Color= White Type= Sorcery Cost= 2WW EX(R)
Text (EX+errata): Each player chooses from the permanents he or she controls an artifact, a creature, an enchantment, and a land, then sacrifices the rest. [Oracle 1999/05/01]
If you control a permanent with more than one type, you can choose that same permanent for more than one of the choices if you want to. [Barclay 1998/06/10] This makes it possible to select a Brass Man as both your artifact and creature, and then select a land and thereby keep only two cards. You probably won't want to do this often, but it is an option. [D'Angelo 1998/08/19]
If you control a permanent with more than one type, you can choose that permanent to be the representative of either type. This makes it possible to have more than one creature (or other permanent type) in play. For example, if you select a Scryb Sprite for your creature and a Brass Man for your artifact, you get to keep both of these creatures. [D'Angelo 1998/07/23]
The permanents to save are chosen on resolution. You do not choose them when announcing the spell. [WotC Rules Team 1998/08/01]
The current player makes their choices before the other player. After both players make choices, all the sacrifices are done simultaneously. [D'Angelo 1998/11/04]
Nothing about all the permanents having to be in play
If it said "target creature, target enchantment, target artifact, and target land," then it would be as Martin said. However, it doesn't say that. All you need to cast a spell is legal targets, and Cataclysm targets the players rather than permanents. Hrm. You are right about the MoM and Pianna. MoM's ended up being rarely cast, and Pianna's bonus is nil with only one creature in play. Here's the changes.
-4 Mother of Runes
-1 Pianna, Nomad Captain
-1 Lieutenant Kirtar
+4 Soltari Monk
+1 Paladin en-Vec
+1 Swords to Plowshares