What does + mean in Arcanum weapon stats?

Weapons in Arcanum are described by two letter stat values. However, the same stat is sometimes given as
number
and other times+number
.For example:
 Mystic gauntlets: AC +4
 Dwarven gauntlets: AC 3
Is there a difference? Is so, what is it?

I believe you mean Mystic Gauntlets AC:(+4). This means that this bonus stat in parentheses is an enchantment. Its effectiveness is determined by your magickal aptitude. The Magic power available stat below gives you what percentage of this stat you get.
Only stats in parentheses are enchantments.