It may still default to a bad starting location. It will be using 1 Megabyte alignment for the starting sector (sector number a multiple of 2048), and depending on how the partitions are currently placed, it might find a gap that it could use.
BTW, any recent version of fdisk would default to using sector units. You must be using a fairly old one.
Also, it is not at all obvious that the 4th partition would be an extended one. You would need that if you envisioned ever wanting more than 4 partitions on that disk, but with just 10 GB remaining, how likely is that? Of course, it that is the way the question is phrased, then that is what you would do.