Hi, I'm the friend.

Yes, my argument was indeed the Game Mechanics section you quoted from page 88, cr4kp0t.
The drugs debate has been houseruled by our GM (i.e. drug effects do stack with augmentations or magic), so that's not an issue from this point of view, really. I am aware of the fact that this is a hotly debated topic, however, but there's no need to go into it; let's just assume that they do for the purposes of this discussion.
There are additional references using words like "most" or "few" in terms of exceeding the 4 IP limit, the strongest one to my mind being this:
Page 144, SR4A:
"Few characters can act in more than four Initiative Passes in a Combat Turn and none under these basic rules."
Now, "basic rules" are not clearly defined, but a mention is made on page 221:
"Artificial intelligences under these core rules are plot devices and NPCs; however, new options are introduced in the advanced rulebooks Unwired and Runner’s Companion."
Then there's this, from page 145:
"Most characters may not act in more than 4 Initiative Passes in a Combat Turn (even if they spend Edge)."
The Increased Reflexes spell on page 208 limits IPs to 4:
"A character can only be affected by a single Increase Reflexes spell at a time; the maximum IPs any character can have is 4."
I'm guessing it's an oversight in terms of page 68. I'd argue that RAW states that the maximum IPs you can ever have in SR4A is 5, but RAI is to limit IPs to 4 unless specifically mentioned in one of the expanded books.