More is finer your material - you need more cement to cover it.
Zone 4 sand is finest sand, its surface area is more, so to cover hole area and to have bonding, you need to put more cement in it.
More fines leads to shrinkage.
You will end up in spending more money for utilizing zone 4 sand.
IS Codes says, use zone 4 sand purely base on your experience and test results.
The concrete made with this sand is more likely to have more issues such as
Strength - due to high water demand
Workability - due to clay type properties.
Finishability - leads to more cracking.
Dusting - due to fines.
To use zone 4 sand, one need to take trials and doing experiment to find right combination to achieve desired properties.