When using a 4 deck view mode with Serato DJ hardware that support 4 decks, you will notice 2 of the 4 decks have a blue border. This is referred to as the active deck indicator. This means that all controls on your Serato DJ hardware, and all Keyboard Shortcuts will apply to those decks which have the blue border.
You can either use your Serato DJ hardware to change the active deck, or you can select which deck is your active deck by selecting either 1/3 or 2/4 in the top left or right hand corners of your screen.
Selecting your Active Deck will determine which deck is displayed when using the 2 deck view mode when you have 4 decks.
TIP: Use the keyboard shortcut ~ (tilde) to toggle between having 1/2 or 3/4 as your active decks.