That is: three fully grown modern languages, and one ancient one.

This page, "North-Sea Germanic Wordlist", is very interesting in that it has, side by side, vocabulary lists for English, German, Old English, and the modern dialect (or language?) of Plattdüütsch--Low German (Plattdeutsch in German). Low German, like its ancestor Old Low German or Old Saxon, is somewhat of an intermediary between English and (High) German. Some words you will notice English and Plattdüütsch have more in common (like 'five' and fiev, as opposed to the nasal in German fünf). Other words Plattdüütsch has more in common with its neighbor to the south (e.g., German Baum and Plattdüütsch Boom versus English 'tree' and Norse tré).

The ancestor of Plattdüütsch, Old Saxon, was spoken both by the continental Saxons and by the Saxons who first migrated to the British Isles. After this separation, however, differences occured: some on their own, some b/c Saxon was influenced by German, some b/c Anglo-Saxon was influenced by Norse. Still, most similarities to be seen between modern English and modern Low German are the remnants of this ancient kinship between these two languages.