why can't I pair my ipad2 with my ipad4?

Pair? What do you mean by Pair?

-Ethan

Bluetooth pairing on the iPad is mainly aimed at pairing devices such as headphones or keyboards, not the pairing of 2 iPads:

iOS: Supported Bluetooth profiles - Apple Support

Note: There are apps that allow participants to connect via wifi or Bluetooth (e.g, GarageBand for iOS) to join the same session or game. Both devices should use the same type of network connection. If using Bluetooth in GarageBand to join a jam session for example, then wifi needs to be turned off & Bluetooth turned on, on both devices.

Unless you hacked it, there is no way to pair two iPads (I don't know what you would need to do that for, anyway). As Jazcan has stated, the only "Pairing" you would need to do is if you were using an app like GarageBand.

-Ethan

it can be done but not out of the box

no services build into the ios support what you wish

a third party app could handle it by PAN profile but it would require an app on each device where the apps are then talking together

and it would also require that keynote has an open API for which third party apps can remote control keynote

You can use one iPad to control a Keynote presentation on another iPad via BT using the Keynote Remote feature. Pairing is not required.

See here: Use Keynote Remote on your iPhone, iPad, or iPod touch - Apple Support