Schengen visa of first entry but not longest stay with 1 day difference

Your stay consists of more than just nights, so from the information given we can't really say.

If you enter France at 5 in the morning on the first day and travel from France to Italy at 22 in the evening on the fourth day of your journey and leave Schengen (Italy in this case) at 5 in the morning of the eight day, you'll have stayed more nights in Italy (as described in your question), but you have been in France for 89 hours but only in Italy for 79 hours.

The Schengen rules say that you have to get your visa from the main destination and in cases where no country can be named the main destination the first country you'll visit. I (but I don't work in immigration) wouldn't consider the nights as important, most likely you'll be asleep and for most people it doesn't matter if they sleep in a French or an Italian bed, but you are getting close to an itinerary where Italy is the main destination (but see my example above) and in that case, you're also getting close to a case where it can be claimed your visa was obtained wrongly, and that could be a reason to cancel it. If you did describe a plan of going to France and Italy and have a reasonable explanation for having changed your plans for one or two days, I would guess it will be allowed.

As I understand it, you have a Schengen visa issued by France for an itinerary covering France and Italy.

• As Henrik pointed out, you are allowed to make minor adjustments to your itinerary after the visa is granted. Changing the transfer between France and Italy by one day would be seen as minor.
• You are not allowed to misrepresent your itinerary to change where you apply, a practice known as "visa shopping." If any immigration official believes that you lied to get your visa, it doesn't really matter if it was a big lie or a little one, you would be considered untrustworthy.

It comes down to timing. If they believe that you made the decision after the application, all is fine.