Yes it is possible, but you have to go through a slightly round-about route. I have made a note to look at fixing it for a future release.
You need to first
mdadm --grow /dev/md2 --size=max
and then try
mdadm --grow /dev/md2 --raid-devices=4
That won't work but will tell you to use "--grow --array-size=" to reduce the array size. It will tell you what size is needed. So
mdadm --grow /dev/md0 --array-size=whatever
mdadm --grow /dev/md2 --raid-devices=4
That should do what you want.
