Mosfet switch

I have a IXFH50N20-ND and just started to read about it, and it looks like all i have to do is apply 4vdc to the gate, or in the picture Pin 4, and that will switch the 12vdc. here is the link
http://blog.savel.org/2007/02/03/switch-load-using-com-port/
and here are the spec for it
http://www.digikey.com/scripts/DkSearch/dksus.dll?lang=en&keywords=ixfh50n20&WT.srch=1
Digi-Key Part Number IXFH50N20-ND
Im only switching 3 amps, but i have a few of these around.
tks
John

4Vdc won't fully turn on that MOSFET. If you look at the spec you will see that 4V Vgs is the worst-case value to generate a drain current of only 4mA.

To fully turn on the MOSFET you must look at the Rds(on) measurement voltage which is Vgs = 10V, so that is the Vgs you should use to use the MOSFET as a low-impedance switch.

If you want a MOSFET that fully turns on at a lower Vgs then you must use a Logic Level type.