If you find the unit cost per soda, you can multiply the cost of a single one by the amount you want to buy to get the cost of that amount.
If 4 sodas cost $6, divide the cost by the amount to get the unit price.
6 / 4 = 1.5
So each soda costs $1.50. This is the unit price.
Now multiply the cost of 1 soda by the amount you want to buy, which is 6 in this case.
1.5 • 6 = 9
So the cost of 6 sodas is $9.
So the answer is that 6 sodas would cost $9, if 4 cost $6.