90 % of DL-4 failure is in the switches which consist of two parts:
the mechanical plunger which in itself has at least 5 separate parts including two springs,
and the tack switch that is hard wired to the PC board.
Its easy to open a dl-4 remove all of the knobs and nuts from the out side and remove the pc board to examine the plungers. Sometimes one of the plunger springs can become miss aligned or fall out, and you might be able to put it back together if its come apart They do wear out over time and are not expensive to replace. You might need to order them through your local line 6 dealer, or perhaps online customer service. The tack switches are also cheap but require soldering to the PC board. They also give out put not as often as the plungers. You might check the archives for more on this subject.