Introduction: 2x2 Piston Door
First off you will need to find the place you would like to have the door
Step 1: Dig It Out!
Begin digging out the hole just like the one you see above.
Step 2: Add Redstone
Fill the bottom with redstone
Step 3: Fill in and Add Pistons
Fill the floor back in and add 6 stickypistons
Step 4: Choose You Block
Now choose the four blocks you would like to have as a door.
Step 5: Finishing Touches
Now just add a lever and fill in the walls beside it and you are done!