Stunning in GURPS is pretty nasty. It can be lethally nasty with human-level combatants. It gets progressively less nasty the more skilled you are.
Being at 1/3 HP or below results in halved Dodge. You could do the same for stunning. Instead of a flat penalty of -4, all of your active defenses are halved (round up). If you are stunned and are already at half Dodge, divide your score by 4 (round up).
This can make stunning much more lethal against high-skill foes. No longer are folks with a DB 3 shield, Broadsword-18, Combat Reflexes, and Enhanced Parry 1 being totally unworried about stunning. That Parry 17 becomes Parry 10, not Parry 13, with this rule. That's assuming you don't halve DB (I don't) but do halve anything that increases your actual score. And it scales nicely to a "normal human" - DX 10, HT 10, Speed 5, Dodge 8, bare handed parry of 3 + DX/2 for an 8 - both become 4s, same as the effect of stunning now.
Cumulative parry penalties are unchanged, and apply after halving.
Why after? Because that's consistent with the rules for Limiting Multiple Dodges in GURPS Martial Arts. That book doesn't say to apply it and then halve for low HP, it just says to apply a penalty. So it stands to reason that you apply cumulative penalties (and those from Feints, Deceptive Attacks, etc.) to your final score. Applying it first would mean that halving your Parry also halves any additional problems you have with your Parry, which is bizzare ("That Feint doesn't bother me as much because I'm stunned!")
Of course, any player worth his knowledge of the rule books will point out that technically, stunning is a -7 to defend - you can't Retreat! This is much more than a -7, since it's 1/2 -3 instead of that flat penalty. This penalty approach naturally makes it more. That's intentional.
When you are stunned, your active defenses are halved, round up, instead of being at -4. This is in addition to the normal effects of stunning. Apply any situational penalties to your active defense aftering halving. If you already suffer from halved defenses, divide your defense by 4 and round up before applying any situational penalties.
That would be pretty lethal. I haven't tried it, but generally I find that "half" or "double" approaches work well in play. You don't have to look them up. You do need to know what your DB is, but that's not an unreasonable extra step to take. It's faster than looking up penalties, and is pretty fair.