Iron has only 2 valence electrons... How did it became four? http://youtu.be/bht9AJ1eNYc?t=24m50s at 24:50 said Iron has 4 electrons, Iron has only 2 valence electrons... How did it became four?
Also... Why magnetic field of electrons don't cancel each other out...?
If the  valence electrons are 2, 4, 6, etc... (even numbers), nothing will remain because half of them will cancel the other half which will lead to zero magnetic electrons.
Valence electrons with 3, 5, etc... (odd numbers), e.g. with 5 magnetic electrons, 4 will cancel each other out, hence only one magnectic electron will remain... so do with 7 VE, 1 VE will remain because Six VE cancel each other out.
 A: The electronic configuration of iron is: $Ar\,3d^6\,4s^2$. the two electrons in the $4s$ orbital are paired, and two of the electrons in the $3d$ orbitals are paired leaving four unpaired electrons.
In general when you are populating the $p$, $d$, $f$, etc orbitals the electrons will only pair up when they have no choice. So for example in manganese where the configuration is $3d^5$ all five $d$ electrons are unpaired.
A: Apart from the fact that the images that the video provides are mostly wrong, it does not talk about valence electrons; it just says 4 electrons. These are from the $d$-orbital. The magnetic contribution of the 2 electrons in the $s$ orbital is zero, as you correctly note.With respect to why 6 electrons in $d$ result in 4 remaining spins, in contrast to your wrong 2,4,6,..-rule, you should check Hund's rule.
Eventually, the presence of these extra 4 electrons is by far not sufficient to get ferromagnetism in solid iron. You may get a rough idea by checking the Stoner criterion, but at the end its quantum mechanics, band structures, etc.
