Not sure on this one but I am confident about it:

On page 20 of the rules about Brain Damage,

The Runner randomly trashes one card from his grip for each point of brain damage done to him, and his maximum hand size is permanently reduced by one card. The Runner takes a brain damage token to track this .

So if the particular piece of ice had a subroutine, "Do 4 brain damage," it would mean that the runner would randomly trash 4 cards and then take one brain damage?

Maximum hand size would be reduced by 4 in your example. From page 30:

Damage, Brain: A unit of damage that
requires the Runner to trash one card
from his grip at random, and reduces his
maximum hand size by one card.

One card randomly discarded per unit and a reduction of maximum hand size by one per unit.

Ok, got it. It just sounds confusing with "Do 4 Brain Damage" sounding like you would take 4 Brain Damage tokens, thanks.

You would take 4 Brain Damage tokens, yes. How else are you supposed to track your new maximum hand size?

To be clear, 4 brain damage means you randomly discard 4 cards and take 4 brain damage tokens. Your new maximum hand size is 1.

The expanded rules on page 20 clearly state:

"The Runner randomly trashes one card from his grip for each point of brain damage done to him, and his maximum hand size is permanently reduced by one card. The Runner takes a brain damage token to track this."

So you get hit for a 4 brain damage blast. You lose 4 cards, but only get one Brain Damage token. Your hand size is reduced by one.

The glossary is so shortened it is confusing.

Fortunately, all 4 cards that currently deal brain damage do so 1 at a time (3 HB ICE and Stimhack). Given the lack of brain damage prevention and the more powerful nature of brain damage (compared to meat and net), I don't see that changing any time soon, leaving FFG plenty of time to clarify.

The glossary clarifies that the wording applies to each unit of damage. Four brain damage reduces hand size by four.

Doc Savage said:

This is incorrect.

4 Brain Damage = 4 Discards from Grip + 4 Brain Damage Tokens

Khudzlin said:

Heimdall does 2 brain damage.

byronczimmer said:

Unless cardgamedb is lying, Heimdall only does 1 brain damage:

The Runner can spend [Click] to break any subroutine on Heimdall 1.0.
[subroutine] Do 1 brain damage.
[subroutine] End the run.
[subroutine] End the run.

Sent this question into FFG and got a response from Lukas….

"The rules are talking about *each time* that the Runner takes 1 point of brain damage. So if the Runner takes 3 brain damage from a single source, he will have his maximum hand size reduced by 3."

So there we go.

byronczimmer said:

I have the card in hand right now, he does 1.

Butaman551 said:

I'm still sort of baffled that anyone thought otherwise. Thanks for posting the clarification!

Re; Heimdall, thank you - I may have entered the data off of an old card image.

Butaman551 said:

Wow, that was a tricky one. Sounded almost like one. Thanks.