Honestly, I think drugs are still pretty useful even when they don't get to break the +4 cap.
Because lets face it, how easy is it to get the +4 cap anyway ?
For most methods if you are picking up a +2 then it is nice if you can pick up another +1/+2 via Drugs & have it stack.
I mean, short of Increase Attribute spells where 4 successes is fairly common on 12 dice, most Cyber, Bio, Chemical, Adept Boosts are all going to give lower bonuses than +4. The spells they are usually temporary, they need to be Sustained via Dice Penalty, Foci, or Quickening.
My thought is that the various options to boost a stat should all stack w/ each other, but, that the +4 limit should be in place.