Yeah, I agree the backpack description might be a bit misleading. "+4 inventory slots" could definitely sound like just an upgrade to your existing capacity. I'll change it to something clearer, like "allows carrying consumables (4 slots)" or along those lines.
As for Winterheart, it does mention "-1 Power" in the description.
Thanks so much for the feedback! Feedback like this is really helpful, since there are things I might otherwise overlook.