$columns = import-csv C:\infile.csv -Header 'column1','column2','column3','column4' -Delimiter ';'

$Result = @()
ForEach($i in $columns){

   $found = 0;

   ForEach($m in $Result){

    if($m.column1 -eq $i.column1){

        $found = 1

        if( $i.column4.length -ne 0 )
        {   
           $m.column4 = $i.column4
        }
        break;
      }
   }

   if($found -eq 0){
        $Result += [pscustomobject] @{column1=$i.column1; column2=$i.column2; column3=$i.column3; column4=$i.column4}
   }
}

$Result | export-csv C:\out.csv