Improving an asset is not expense. It is part of basis in that asset. If the property was not in use at the time, you have new accrued invested value in that asset, without income or expense from that property. That is, for instance, how a Flipping activity is done.

If this is a rental property they are keeping, then you have to determine if the property was available for rent while also being rehabbed.

Expenses are things not represented in the rehab, such as if they still paid for yard work. Ongoing costs such as this are not part of investment. New windows are not expense; that value still exists, right there in those windows.

It's not Sched C unless your taxpayer client is in the business, such as buys, rehabs and flips, all the time. Even then, it still is not expense if that is part of the invested costs (new basis) in the improvements.

"do I wait to use From 4797 and fill in the Gross Sales Price, Cost & Expenses etc?"

Here is the overview of what typically happens:

You buy property, which is part of basis. You invest further via the improvements. That can cross year ends, so there is nothing to report, yet. Once it is on the market for sale, you have the total invested cost as Basis, you have the expenses of the sale (commission, closing fees, etc) and you have the Sale Price. The math gives you profit/loss.

That is either reported by a business person such as a carpenter might do this in their idle time = income from business operations. The property is their inventory.

A homeowner handyman might buy the house next door to flip, and that is the first and only one or once every 3-4 years = more like an investment, not business income.

Facts will determine what applies.