How to add a drain pipe to a concrete block wall covered in insulation?
They make shelf brackets that have a U shaped hook on the end away from the wall.  I've seen them in a variety of sizes, down to about 6" (don't know about 4").  That would be pretty easy, just space some of those out at the height you want, then lay the pipe in the "trough" created by the hooks.

I guess they're called shelf and rod brackets.  Here's a link, though it's probably too big for what you want.

This one looks perfect for what you want, but I don't see how to buy it..

Do a google search and a google image search for "rod bracket" "shelf and rod bracket" you'll see ones made for closets, but also others for curtain rods, which tend to project less from the wall.  I did a search for '6" rod bracket' and saw many potentials, at a range of price points.