How can I make a 2x2 piston door that is flush with both the inside and outside walls with no visible redstone?

Solution 1:

Sound's like you're asking about a hipster door, seen in this video by Reversed Gravity. The idea of these is that everything is flush with a 1 wide wall, where all the restone is beneath the floor. This design is showcased as 1-wide, but it can be expanded infinitely due to its tileable nature

As far as I can tell, this is the most recent and compact design, but feel free to correct me.

Edit:

As per Fabian's request, here's a screenshot with a slightly modified design so it's 1.15 compatible:

Two things to note:

• The red arrows on the blocks are a tweak from a texture pack, they are observers with the arrows telling you which direction they should be facing. The same with the sticky pistons
• The observer facing down on the bottom left has a piece of redstone dust on it, this is a modification of the original design because of a change that stopped powered blocks from updating observers