I remember that in 3rd edition, one of the balance points keeping mages from decking themselves out like xmas trees with numerous foci was that an active focus or spell lock was one of the few ways an astrally projected mage could channel a spell into the physical realm.

I have been looking to see if this is still the case, but I cannot find any rules mentioning this in 4th edition. Can anybody confirm/deny this and provide a page reference?

It cannot be done. There is no page reference because mention of its existence is never made.

There is absolutely no way for a projecting mage to cast a physical spell.

Grounding spells through foci or other astrally active things was actually a 2nd Edition rule that was removed in 3rd Edition.

Foci are still limited in SR4. They are still astrally visible, dual-natured when active, and enable tracking the mage through the astral link. There are limits to how many foci a mage can bind, how many can be active at once, and, most importantly, only one focus at a time can add to any dice pool.

Furthermore, availability rules sharply curtail the force of foci at char-gen, and unlike SR3, foci cannot be used to help soak Drain (a few used to have this ability, but the erratas have changed them). Focus addiction is available as a negative quality, and there are rules for imposing it for excessive foci in the Digital Grimoire.

Generally, SR4 characters tend to have fewer/lower-rated foci than their SR3 counterparts.

I am not sure, but when they are active can't foci be attacked by an astral form? Not that a spell can be "ground" through them, but the foci itself, with the end result being that the foci is "turned off." (Which would just make the foci's mage have to "turn it back on" with a simple action (I think).

The projecting mage could be the spotter for a ritual spellcasting party who's using a hair/DNA sample (or something else left by the target) as a link through which the ritual spellcasting will be cast. Along with the physical link, the group needs a spotter who can see the target and who's also part of the ritual spellcasting party. The spotter can be viewing the target while astrally projected.

That is correct. A focus is dual natured and exists in both real and astral space at the same time. Active foci cannot be taken through a mana barrier -- if a normal person with an active focus in his pocket attempts to walk through a mana barrier, either the barrier will crash or the focus will be deactivated (the normal person won't have any problem walking through the mana barrier, presuming that they aren't astrally perceiving and thus temporarily dual natured themselves (or a shapeshifter or drake, who are always dual natured). Since an active focus is dual natured, the focus itself can be attacked by a projecting mage (or a spirit).

i think grounding as a primary limiter on foci use was removed in favor of focus addiction. That is, use a total rating of foci higher then the magic rating and one risk burning out ones magic rating.

Turning off a sustaining focus with Increase Reflexes means the first pass of the following combat TURN they get to recast it as a complex action and then let the focus sustain it. Not sure if its an action to make it an active sustaining focus or not.

The Disrupt [Focus] spell from Digital Grimoire is especially nasty because the focus cant be made active for several combat TURNS after its shut off on a decent roll. Every possessor, projector and free spirit should have Disrupt [Weapon Focus] to make the adepts without killing hands cry.

And they wouldn't get the bonus from their Increase Reflexes focus until the <i>next</i> combat turn after they activate it. So, having a spirit disrupt the focus from the astral plane then immediately manifest and attack would be a good move.