The chance of having a player being a smurf is expressed by:
s/P
Where s is the number of smurfs in your division, and P is the population, or total number of players in your division.
The chance of having a smurf on your team is expressed by:
n x s/P
Where n is the number of players on a team who are not premade with you, since anyone who is with you, you would know if they were a smurf.
Assuming you're solo queuing, your chance of having a smurf teammate is 4 x s/P while the enemy is 5 x s/P.
To put it in simple terms, the enemy gets to roll the dice 5 times while you only get to roll it 4 times. Not sure how you came up with 1/4 against 1/5.
With that said, my first point still stands: the chances of having a smurf is so negligible that a marginal difference of 0.2 is not going to make much of a difference on the rolls.