According to the wiki the Improved Parry feat just adds +4 to your parry opposed rolls (essentially your skill) when using the skill.
I don't think it makes much sense for the +4 skill bonus to be invisible. Either change it to just add the +4 skill bonus to your sheet or if possible adjust it to give the +4 bonus to riposte attack rolls (which is what the feat in game still says it does, but it's just +4 to the skill roll, not the riposte attacks).
From the Wiki:
The bonus is added directly to the parry skill, similar to skill focus, but with a slightly larger (and stacking) bonus.
I believe it already does add it directly? (https://nwn.fandom.com/wiki/Improved_parry )
Interesting! I will have to check if it's true, I assumed it was just added to the opposed roll.