Can I switch control decks to DECK 3 or DECK 4?

You can switch control decks when using Performance mode in rekordbox (when Control mode is on) as follows:
To switch the deck, touch [DECK 1] or [DECK 2] in the deck information section on the Playback screen (waveform screen).

Note: It's not possible to switch decks when you're playing tracks on a USB device or using rekordbox Export mode with a Link Export connection.