Get children of specific parent
You need to get the parent entry model before requesting its children with the getChildren method. craft.entries.id(4) only creates a criteria model prepared to query for entries matching your criteria. You now have to use the first method to query for an entry model.

{% set parent = craft.entries.id(4).first() %}

{# Check if it indead returned an entry model, could also return nothing #}
{% if parent %}
    {% set children = parent.children %}
{% endif %}