I'm going to try to explain how this works in my head, but I'm not sure if it will make sense.
I also might be completely wrong.
But I figure this is the math behind it:
1 card means that each hit has a 10% chance of gaining 5% sp.
4 cards would mean that each hit has 4 10% chances to gain 5% sp.
40% would mean that 4 out of every 10 hits should steal 5% sp.
But 4 x 10% would mean that each hit has 4 1/10 chances to steal the sp. 4x0.1=0.4, or 40%
So it should end up being the same, whether they stack indefinitely or check 4 times.