Building upon a recent bug report (Ravager's Dreadgaze always rolls 4% cooldown) as well as some other mentions, I thought I would check and see if other items exhibited this same 4% roll all the time.
From searching in GT, I found that any item with the “+3/5% Skill Cooldown Reduction” will never roll any other value other than 4%. There is simply no variance with this stat whether it is present on a helmet, weapon, shield, off-hand, or amulet.
I searched countless items with that CD reduction stat using the Grim Dawn Seed Finder to see if these would be listed as a changeable attribute and no item with that +3/5% listed the option to change. Every other range for CD reduction has variance listed as an option that can roll different using this tool.
I also crafted countless items with the +3/5% range in GD Stash and these items would only ever be 4%. All other items I crafted with any other CD reduction range had the correct variance as expected.