[orgo814]

If looking at a very simple sigmatropic model: 1,3 migration of hydrogen in propene by sigmatropic rearrangement... Why is this considered a 4 pi transition? It only has two pi electrons. This isn't cationic or anionic. Please clarify

[phth]

Whether it is a sigmatropic, a radical, or a cationic isomerization is up to debate because it depends on how it's treated, what it's in etc.
However, it's a rule that is designed to govern what way the orbitals rotate because of symmetry between the LUMO and HOMO of the transition.  For example diels alder, that means the diene and the dieneophile.  For a transition that you're talking about it is a 4n transition aka mobius transition.  it requires the diene to become energized, e.g. a photon energetic enough to excite it to the next orbital(HOMO-->LUMO).  For the 4n system it is always higher in energy because the LUMO is required to react.  next in line is the 1,5 hydride shift.  this is very common and drives cyclopentadienes into conjugation (by the thermodynamic favorability of the more substituted double bond). It is a 4n+2 (huckel transition), and these are always lower in energy because they dont have to be energized first.  anion rearangements are perfectly fine, but how energetically favored is a different question.