Here is the list of all the English words with 20 letters containing only letters J, N, V and X grouped by number of letters:
All fields are optional and can be combined.
You can search for words that have known letters at known positions, for instance to solve crosswords and arrowords.
Lots of Words is a word search engine to search words that match constraints (containing or not containing certain letters, starting or ending letters, and letter patterns).
You can use it for many word games: to create or to solve crosswords, arrowords (crosswords with arrows), word puzzles, to play Scrabble, Words With Friends, hangman, the longest word, and for creative writing: rhymes search for poetry, and words that satisfy constraints from the Ouvroir de Littérature Potentielle (OuLiPo: workshop of potential litterature) such as lipograms, pangrams, anagrams, univocalics, uniconsonantics etc.
Please note: the Wiktionary contains many more words - in particular proper nouns and inflected forms: plurals of nouns and past tense of verbs - than other English language dictionaries such as the Official Scrabble Players Dictionary (OSPD) from Merriam-Webster, the Official Tournament and Club Word List (OTCWL / OWL / TWL) from the National Scrabble Association, and the Collins Scrabble Words used in the UK (about 180,000 words each). LotsOfWords knows 480,000 words.