How to install low voltage bracket through a drywall block?
It was not particularly clear from your posting just what this 1x4 is in the wall. If it was flat to the back side of the drywall surface and went up at an angle in one direction and down at an angle in the other direction then this is a big problem. In this case this 1x4 is a cross brace installed in notches across a batch of studs. It's purpose is to triangulate with the studs to stiffen the building structure and keep things square. This would have been something that should not have been cut. Instead you should have moved your hole over and patched over the first  hole.