"A relation is in fourth normal form if it is in BCNF and contains no multivalued dependencies." (S. Sumathi & S. Esakkirajan, "Fundamentals of Relational Database Management Systems", 2007)
"A table is in 4NF if it is in BCNF and contains no unrelated multi-valued dependencies." (Rod Stephens, "Beginning Database Design Solutions", 2008)
"A table is in fourth normal form (4NF) if and only if it is at least in BCNF and if whenever there exists a nontrivial multivalued dependency of the form X->>Y, then X must be a superkey in the table." (Toby J Teorey, ", Database Modeling and Design 4th Ed", 2010)
"In relational theory, the fourth of Dr. Codd’s constraints on a relational design: No column within a primary key may be completely dependent on another column within the same primary key." (David C Hay, "Data Model Patterns: A Metadata Map", 2010)
"A table is in 4NF when it is in 3NF and contains no multiple independent sets of multivalued dependencies." (Carlos Coronel et al, "Database Systems: Design, Implementation, and Management" 9th Ed., 2011)
"An entity is in fourth normal form (4NF) if and only if it is in 3NF and has no multiple sets of multivalued dependencies." (Craig S Mullins, "Database Administration", 2012)
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