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!