mATX boards are just three expansion slots shorter, and originally allowed a weaker power supply unit.
The latter point is of course history, unless you go by the spirit of what mATX was meant for (cheap low end boxes)
and use a low end processor.
So a P4 mATX board in a standard ATX case, with a capable power supply unit, is not a problem. You'll
just have lots of empty space and three unused slot brackets at the bottom.
edit: Forgot to say that the mounting holes, connector panel position, and power supply connections are the same.
regards, Peter