Ok, a little bit more clear now, thanks.
I was thinking about putting on all output pins of the arduino an small transistor.
If all outputs would be high, the Arduino can't power everything I think. To be sure of this, I tought, why not boost it with an transistor.
Can that be a problem?
How much current wil an mosfet use normally? 2mA or something?
Good idea about the resistor.
Any other ways to make it an safer switch circuit?
Some spook about fuses and zenerdiodes and resistor on the gate, don't know what it all does, or if it's usefull.
But I like to be on the safe side.
Ok, I now know it is thermal watts. But how much would the IRLZ34NPBF be capable of switching, and how much thermal watts would make for example the 48v 4A?
And also, the 5v and ground from the Arduino will be provided by a long (around 5-20meter) Cat6 cable. That would be ok right?