Copying a lot of code to filter four PHP arrays
comments should explain what the code does, at least the complicated blocks, such as the array merging, and/or a comment to describe the whole thing
Be consistent with naming things: you use camelCase sometimes (e.g. userArray) and underscores other times (e.g. field_value_gender). IIRC, underscores is the PHP style, so you could go with that, but the important thing is to be consistent.
Same goes for your filter parameter names: service_filter, region_filter, postal_code_filter, gender_filter
When you have places where you repeat code, use a function:

function build_array(post_field, user_field) {
    $post_value = $_POST[post_field];
    $user_value = get_cimyFieldValue(false, user_field, $post_value);
    $arr = array();

    foreach ($user_value as $value) {
        $arr[] = $val['user_id'];
    }
    return $arr;
}

build_array('service_filter', 'service');

But really, you probably don't need that. Are you trying to get a unique list of user IDs? Can you use one array and call array_unique on that? Or, are you trying to get duplicates? Or both? Either way, I really doubt you need all that merge/diff/unique'ing. For example, you could keep an array of user_id =&gt; times_seen and then loop through that to get entries with times_seen > 1 or whatever.