int GetRowIndexPerPage(int currentRowIndex, int numberOfRowsPerPage)
{  
    return (currentRowIndex % numberOfRowsPerPage) + 1;
}

var myResult = someTable.Select((r, i) => new { Row = r, Index = GetRowIndexPerPage(i, 4) });

var myResult = someTable.Select((r, i) => new { Row = r, Index = i });

.MainTablePreferences(table =>
            {
                table.NumberOfDataRowsPerPage(5);
            })