If I use a 19V 4.7A supply to a MAX 250W ATX power supply am I an idiot or should it be safe enough?
If I understand this correctly, you intend to use this "PeakPSU", apparently a cheap PicoPSU knockoff:



...which is a board with a bunch of DC-DC converters intended to generate the usual voltages to power a PC motherboard from a 19V laptop power supply.

The "specification":



Note it doesn't spec all the stuff you'd really like to know about a switching converter or power supply, like how much current each output can provide, output voltage accuracy and ripple, efficiency, acceptable ambient temperature, etc. So you can't know if it will make your motherboard happy.

Also it seems full of cheap "general purpose" capacitors of unspecified origin. Maybe they're the low-ESR models, but there is no way to know. If they are not rated for the ripple current they will take in use, then they will die.

Maybe if you power a motherboard that only requires 30W they will die a bit slower... but they look disturbingly similar to the caps on these ubiquitous fake "LM2596" modules that are everywhere these days, and these are trash. They could at least have invested in a bit of purple paint to make them fake OSCON's.

Why not buy the genuine instead of the knockoff? It will work with your 19V power supply (it says "12-32V input") and it comes with full specifications so you can check the current on each output will be what your motherboard requires, a manual, etc... and it's almost the same price!