How do you loop through $_FILES array?

Your example form should work fine. It's just that you are expecting the structure of the $_FILES superglobal to be different than it actually is, when using an array structure for the field names.

The structure of this multidimensional array is as followed then:

$_FILES[fieldname] => array(
    [name] => array( /* these arrays are the size you expect */ )
    [type] => array( /* these arrays are the size you expect */ )
    [tmp_name] => array( /* these arrays are the size you expect */ )
    [error] => array( /* these arrays are the size you expect */ )
    [size] => array( /* these arrays are the size you expect */ )
);

Therefor count( $_FILES[ "fieldname" ] ) will yield 5.
But counting deeper dimensions will also not produce the result you may expect. Counting the fields with count( $_FILES[ "fieldname" ][ "tmp_name" ] ) for instance, will always result in the number of file fields, not in the number of files that have actually been uploaded. You'd still have to loop through the elements to determine whether anything has been uploaded for a particular file field.

EDIT
So, to loop through the fields you would do something like the following:

// !empty( $_FILES ) is an extra safety precaution
// in case the form's enctype="multipart/form-data" attribute is missing
// or in case your form doesn't have any file field elements
if( strtolower( $_SERVER[ 'REQUEST_METHOD' ] ) == 'post' && !empty( $_FILES ) )
{
    foreach( $_FILES[ 'image' ][ 'tmp_name' ] as $index => $tmpName )
    {
        if( !empty( $_FILES[ 'image' ][ 'error' ][ $index ] ) )
        {
            // some error occured with the file in index $index
            // yield an error here
            return false; // return false also immediately perhaps??
        }

        /*
            edit: the following is not necessary actually as it is now 
            defined in the foreach statement ($index => $tmpName)

            // extract the temporary location
            $tmpName = $_FILES[ 'image' ][ 'tmp_name' ][ $index ];
        */

        // check whether it's not empty, and whether it indeed is an uploaded file
        if( !empty( $tmpName ) && is_uploaded_file( $tmpName ) )
        {
            // the path to the actual uploaded file is in $_FILES[ 'image' ][ 'tmp_name' ][ $index ]
            // do something with it:
            move_uploaded_file( $tmpName, $someDestinationPath ); // move to new location perhaps?
        }
    }
}

For more information see the docs.

just rename your fields this way

Main photo:   <input type="file" name="image1" />
Side photo 1: <input type="file" name="image2" />
Side photo 2: <input type="file" name="image3" />
Side photo 3: <input type="file" name="image4" />

and then you'll be able to iterate it usual way:

foreach($_FILES as $file){
  echo $file['name']; 
}

Short function to rebuild $_FILES['files'] to some more expected structure.

function restructureFilesArray($files)
{
    $output = [];
    foreach ($files as $attrName => $valuesArray) {
        foreach ($valuesArray as $key => $value) {
            $output[$key][$attrName] = $value;
        }
    }
    return $output;
}