Rank 4 in a spell school allows you one master spell. Since there is only one "powerful" spell in each spell school, it makes sense to get rank 4 in each of the elemental schools.
Uh, no.
Let's do the math here. There are no bonuses to skill-granting abilities. You get 5 ability points on character creation. Levels 1-5 give you 1 ability point per level (5). Levels 6-11 give you 2 ability points per level (12) Levels 12+ give you three per level, but let's set the cap at level 20 (27). So that's 5 + 5 + 12 + 27 = 49.
It takes 1 + 2 + 3 + 4 = 10 points to reach rank 4 in one ability. So to take 4 elemental schools up to Rank 4, that consumes 40 of the 49 possible ability points from leveling, leaving you exactly NINE (9) to use on anything other than skill schools.
That means that if you go for the 4 schools @ 4 build, you won't be able to get anything else above Rank 3 (and even then you'll only be able to get one thing to Rank 3).
Is it technically possible to get 4 schools up to Rank 4? Yes. Does it make sense to do so, at the expense of everything else (especially protection from status effects)? ... In my opinion, no. At the very least, it is nowhere NEAR as much of a "no-brainer" choice as you imply it is.
(And YES I know about
the Talent trade and the pair of +3 ability point books.
It still doesn't change my opinion on that.)