With physical and mental attributes, yes, but with Initiative, this is not exactly the case.
Yes. The limit only apply to augmentations (cyberware and bioware) and only apply to physical and mental attributes. There are also specific spells and adept powers that also fall under the same +4 limit that augmentations use, they all explicit mention when this is the case.
It does not seem as if the reaction bonus to improved reflexes adept power or drugs fall under the +4 limit that augmentations use since they are not augmentations and they don't explicit mention the limit...
What I meant was that you still can't for example get +5 reaction if you combine increase attribute reaction with 3 hits and improved reflexes 2 adept power (in this case increase attribute reaction spell will get a forced limit to a +2 reaction increase and you would end up with a +4 increase to reaction).
I'm not sure how a power that provides +12 Initiative for 1.5 power points is "99% worthless." The drain can be an issue, but on a PP to efficiency rating Adrenaline Boost is awesome.
The bonus to initiative that you can activate every combat turn with a free action also stack with initiative bonus from reaction enhancers augmentation, wired reflexes augmentation, adrenaline pump augmentation, suprathyroid gland augmentation, increase reflexes spell, increase reaction spell, increase intuition spell, improved physical attribute reaction power, cram drug, jazz drug and kamikaze drug...
...actually, the only two enhancers that adrenaline boost is incompatible with is synaptic booster augmentation and improved reflexes power (sure, they are common - but not sure I agree the two make adrenaline boost 99% useless?)
Also, if you have 1.5 PP worth of adrenaline boost you will can still use it to improve your initiative when you activate it even if you have synaptic booster augmentation or improved reflexes power (if you rolled bad on your initiative dice - on rank 1 it will always improve your initiative even if you rolled 6 on your extra initiative dice, on rank 2 as long as you roll 9 or less on your two extra initiative dice and even on rank 3 as long as you roll 8 or less on your three extra initiative dice). It is not compatible so it does not stack, but being incompatible does not mean you can't activate both powers at once. You just need to pick what piece of gear to use at what time.
(if incompatibility have the same meaning for spells as it does for augmentations;))