My take on it is that yes, you should place the shadow on IV.

Under the existing conditions, you only have two stops difference between the shadow and hightlight. Placing the shadow on III will,indeed, put the highlight on V. To develope V to VII is possible - but it is not a standard +2 scenario.

The lower you are in the scale, the more development you need for any relative difference. Moving Zone VI to Zone VIII takes less extended development than moving Zone V to Zone VII.

Of course you need to be mindful of where your particular film/developer combination maxes out and the curve shoulders off. Expanding the Zone VI are to VIII may depress some of the highlight separation that you are looking for.

Another side note - I took a workshop ( many years ago ) from Oliver
Gagliani. He was quite pointed in saying that our modern films don't offer as much expansion possiblilities as older films did.

His approach was to let his negs sit in a nitrogen burst development setup for hours at times. He held that " back in the day" there was enough silver in the films to allow for extreme expansion.

I honestly don't know what the limits of expansion are these days. Technically you should be able to shoot with your shadows in II or III and expand as you like.

At some point, the developer will be exhausted and/or the silver will be reduced as far as it will go.

Bottom line - the previous post was probably right - try it - and let us know

Later,
Terry