What bugs me about this idea is that it breaks how the game works so soon since it's release.
One thing I love about PF2 and what made me use it as my main system is it's organization, all caster have universal mechanics, all martials heavy had patterns on their feats and abilities, they all are still unique and fun, but they followed patterns.
The magus and summoner breaks this idea entirely by creating a whole new system just for this two classes, I mean, if you guys wanted to make halfcasters, you could have made it with champion and rangers as well.
And a simple solution to it is just use archetype spell system, this way they'll not be great at spellcasting, but they'll cast nonetheless
I just think this 4 spells mechanic is a red alert for me, if they start putting new subsystems and different rules in a less then 1yo game, imagine what the game will look like in 4 or 5 years.
i'm just afraid it becomes 3.5 all over again.