So I got in my car this morning and noticed my brake pedal was mush and going all the way down to the floor. Brake reservoir is empty. I'm confident that I have a leak. A few questions.

What is the best possible way to test for a leak?

After finding and fixing the leak can I use DOT 4 fluid?

Any feedback would greatly be appreciated.

Thanks, Phil

 

Get the car up and look for wet spots, check the rubber lines first, if you have drums in the rear, check the wheel cylinders.
It could also be a failure of a line due to rust or damage. All it takes is one kamikaze rock to nail a brake line and take out the system.
DOT 3;4;5.1 are compatible.
The only issue you may have is since the pedal was floored and if the MC is not new the seals may be damaged inside the MC from being overstroked.
Also a change in brake fluid type, especially if it has never been changed or is not normally flushed, can fail a working MC seal by swelling/softening the seal causing it to fail.