These proceedings containall the papers that werepresented at the 4th Inter- tional Conference on Language and Automata Theory and Applications (LATA 2010), held in Trier, Germany, during May 24-28, 2010. The scope of LATA is rather broad, including: algebraic language theory; algorithms on automata and words; automata and logic; automata for system analysis and program veri?cation; automata, concurrency and Petri nets; cel- lar automata; combinatorics on words; computability; computational compl- ity; computer linguistics; data and image compression; decidability questions on words and languages; descriptional complexity; DNA and other models of bio-inspired computing; document engineering; foundations of ?nite state te- nology; fuzzy and rough languages; grammars (Chomsky hierarchy, contextual, multidimensional, uni?cation, categorial, etc. ); grammars and automata arc- tectures; grammatical inference and algorithmic learning; graphs and graph transformation; language varieties and semigroups; language-based cryptog- phy; language-theoretic foundations of arti?cial intelligence and arti?cial life; neuralnetworks;parallelandregulatedrewriting;parsing;patternmatching and pattern recognition; patterns and codes; power series; quantum, chemical and optical computing; semantics; string and combinatorial issues in computational biology and bioinformatics; symbolic dynamics; term rewriting; text algorithms; textretrieval;transducers;trees,treelanguagesandtreemachines;andweighted machines. LATA 2010 received 115 submissions, many among them of good quality. Each one was reviewed by at least three Program Committee members plus, in mostcases,byadditionalexternalreferees. Afterathoroughandvividdiscussion phase, the committee decided to accept 47 papers (which means an acceptance rate of 40. 86%). The conference program also included four invited talks.