We would recommend you to create a sitemap, no matter how many pages your website lists.
If a site is well inter-linked, a sitemap will not give Google any new URLs. BUT it will still be useful for suggesting indexing priorities.
For example if Google decides that it will index only 2 of your 5 URLs (that is quite an issue especially for new sites like yours), the sitemap will point out which ones you consider to be the most important.