Third Generation (since mid 1990s)
Object-oriented, object-relational, and extended-relational databases aim at an appropriate integration of programming languages and databases. The object-oriented data model does not need to follow the first normal form rule of the relational model: A table cell must contain a single, unstructured value.
- Object-oriented data model
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It is a system offering DBMS facilities in an object-oriented programming environment.
Data is stored as objects and can be interpreted only using the methods specified by its class.
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Fourth Generation? [since late 2000s (decade)]
NoSQL data model -
Much of current data processing requires horizontal scaling, faster speed, and processing different kinds of data.
A NoSQL database is to meet the requirements by providing a simple and efficient mechanism for data storage and retrieval.
The approach it uses is different from the one used by a relational database, a general-purpose data store.
Whereas a NoSQL database is normally a key–value store for simple insertion and retrieval operations. Check the NoSQL forecast: people who are looking for NoSQL are doing it mostly because it’s still “hype.” 
