is there such a thing as an extension for a 4" round j-box?

[Lynden]

is there a cover plate that goes over that?

Yes, many different styles. For the mud ring I linked, one option is a 5" diameter blank cover plate. The hole spacing is 2-3/4".

http://www.homedepot.com/p/Hubbell-TayMa...over+plate

If you don't want to see the holes, you can use this type of cover plate.

https://www.cesco.com/images/products/Ar...CP3540.jpg

[rudedan60]

The Simple solution is make your own. Just get a round plastic one and cut the back off to the correct depth.  That way you have custom depth one.  Did this for a friend that dry walled and insulated a cinder block house. The boxes were n the brick.  Dry Waller used mud to mount them in place and we extended the wires.  Worked great. Dan

[EricU]

the borgs have 1/2", I'm going to see how bad that is