You can't just make that statement. If it were done that way you would need to know the load current and the current would need to remain constant. Additionally you don't mention AC or DC. Your question is open.
first off i have a sound to light unit 230V ac i'm trying to mod the circuit to work for led's so i have replaced the triacs with some logic fets but i do not want to use a ac 12v supply for it but on the board befor mod it have a 4k7 resistor 10watt i wanted to know what the output power after the resistor unfortunatly my meter has dodged out so i can not get the right voltage to the chip so i know what voltage reg to use
It sounds like your sound to light unit is what is commonly called a color organ. The problem is that the voltage drop across your 4700 Ohm resistor will be a direct function of the current flowing through it. That current will be a function of the load. This would go better if you could post a drawing. A color organ uses triacs generally which have AC applied to their gates. Then we get into logic level mosfets which is where I am lost. Attached is a basic image of a 3 channel color organ using SCRs to drive incandescent lamps. Is what you have something like that?
Dropping mains voltage with a resisitor is a lousy way to do it. Only a super cheap piece of unsafe chinese crap would use that method. Use a step down transoformer that outputs a safe DC voltage and a regular, if required.