Greetings, everybody. Here's a dual-sided 4x5 door I made that I decided I'd share with the community. It was built in v1.8.2 and should work fine to this date. If the 1.9 snapshots break it, I'll make it a point to fix it accordingly.

The video included provides a tutorial on how to build the door, but there is also a world download available if you prefer that. I've also included a door-opening mechanism in the world download (specifically the one I use) and it is also showcased at the end of the video.

Door works fine in any orientation.

While outlined in the video, here is some useful information I'll be typing in here for reference:

Dimensions:
12x3x14 (504-block volume)

Required Materials:
65 Redstone
66 of any block
21 of any slab
40 Sticky Pistons
36 of any block (for the appearance of the door)
18 of any block (for the walls when the door is retracted)
7 Redstone Repeaters
6 Redstone Comparators
6 Droppers
6 Hoppers
6 of any block/item (for the droppers/hoppers)
8 Redstone Torches
2 of any light-emitting block (sea lantern, glowstone, jack o'lantern, torch — redstone lamps not recommended)

For optimizing the door...
-1 of your 66/any block
-2 of your 18/any block for the walls when the door is retracted
4 Furnaces
+4 Redstone
+3 of any slab
2 Activator Rails (powered works too)
+2 of any light-emitting block