I presume the RAI is that +4 Dice would be the cap regardless of what kind of Foci, FWIW.
I have heard SSDR say that as well, but that is not how I read it at all. Quick run down:
Attributes say: "Various things modify attributes, such as spells and gear, but a character’s adjusted attribute can never be higher than their current attribute rank +4.".
Skills say: "They can be modified by spells, gear, and other effects, but their augmented increase can never be more than +4."
Nothing else in the entire game is
generallyrestricted in stacking other than Initiative, but some
specific things state they will not stack (such as muscle toner and a suprathyroid gland, and only being able to apply one foci to a given dice pool).
Foci say things such as "A centering focus adds its Force to the magician’s initiate grade when they use the centering metamagic on Drain Resistance tests.", and "Spellcasting foci add their Force to your Spellcasting dice pool as long as the spell is in the same category as the focus."
My point:
Foci do not add to a skill - they add to a dice pool. You could have a mystic adept with Magic 6 + Spellcasting 14 (with adept improved ability) + a foci on top of that, because no foci just flat add to a skill.
If the intent is that no
dice pool can be modified by more than +4 then all three of those sections are in dire need of a re-write, because that is not at all what it says.