You are correct...but you get to raise ONE spell by 20 points...
A Diedne with a (say) 10 low art, gets to add 10 to FOUR spells..
If he has a low art of 15, its +15 to FOUR spells.
So you get to add 20 to ONE spell (or 5 to four spells), or he gets to add 40 or 60 to four spells.
The more powerful the Diedne, the more profound the effect. Life Linked stays the same....