Hi there!
Total Defense is when a character wants to use his action to brace for impact/raise his shield instead of doing something else to gain +4 to Defense for a round in D&D and Pathfinder. How do you guys handle this action during an ICRPG adventure? Do you think +4 is too high? Do you ask prerequisites of your players before they can take that action? Do you have a cool alternative to that rule?
Thanks! Have a good day! 
Two options come to mind here:
Simply make a Defense roll against oncoming attacks for the round
Add +3 Defense (essentially making the opponents attacks against him HARD), cap it at 20
Maybe they have to forfeit their move since they are hunkering down and bracing for an oncoming attack.
I have not tested these, so take from them what you can.
I like 1. Also, I’d say if carrying a shield, with this action a PC could by sacrificing movement and action become immune to missile attacks coming from one hemisphere (except in the case of bullets and boulders, etc.)
When this has come up in my ICRPG game, I add 3 to their Defense or make their save Easy.
The +4 sounds good if you sacrifice both move and action, but if that works then a +2 could work for sacrificing only your action.
You can do that, but typically, your get an action and movement. If you don’t use your movement, you don’t get a bonus to your action.
But, maybe at your table your players get extra oomph to their attacks and spellcasting if they don’t use their movement. IDK
I like all of your suggestions, but this one sounds pretty fair: either +2 or +4 depending on what the character is willing to sacrifice and he needs a shield or some other ward to take te action! Nice! Thanks!
I like the idea of +2 for a going “on the defensive” action, where a character in melee is using a weapon, a shield, or their hands to only make sure they do not get hit. I would only apply this against ranged attacks if the character had a shield.
You could also have a +2 “defensive movement” action where the character is trying to move in such a way that they are avoiding incoming attacks. This could work against ranged attacks or melee. If the player is trying to get somewhere and not stand relatively still, you could reduce the distance traveled if the player is running a serpentine pattern against ranged attacks (LoL). (Insert Serpentine! Serpentine! meme here)
I know this isn’t a simulation, but I teach HEMA and Filipino sword fighting, and we cover being on the defensive through both the use of weapon and / or shield as well as movement (but usually both tool and movement, though each work separately, of course). I can sit “toe to toe” with an opponent and focus my actions not on attacking but specifically on making sure I am not getting hit. I can also play Muhammad Ali and just use footwork and body movement and position to make sure I am safe from incoming attacks. Occasionally I use both as a way to set up my own attack. So if you use Both, you could have a +4 option against melee attacks, and if you had a shield, you could have a +4.
This is a long winded answer to say, I too like the +2, +4 thing, I just add an additional +2 option…
I would use the Easy/Hard modifier. If the enemy attacks you, it is Hard, if you roll Defense, it is Easy. Something in those lines. 
What’s nice about EASY/HARD is you bypass the +20 limit on DEFENSE. So you’d either have to rule that the +20 limit applies to E/H or it is a special bonus. Gives some flexibility.