Your shaping will still be carried out at the stated length measurements buy, you may not need to do them as often.
Let's say the pattern says to use yarn that is 6sts=1".
You're using a yarn that gives 4sts=1".
The patern says to decrease 1 st, 6times (6sts or 1" total).
You'll want to decrease the 1" but will only have to decrease 4sts to do so.
Therefore, you would decrease 1 st, 4times (4sts or 1inch) over the given distance.