The Fourth Style, also called by Mau the “Intricate Style”, can be seen as a combination of the other three styles.

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Fantastic
It is also defined as a “fantastic style” because it is heterogeneous and uses elements from all of the previous styles; it is a mixture of the features of the artistic sensibilities that came before.
The Fourth Style in Roman wall painting is usually less ornamented than the previous one, it can be considered a baroque tendency as a reaction to the Third Style‘s mannerism. The style was way more difficult to understand, it revives large-scale narrative painting and panoramic vistas even if it wanted to retain the architectural details of the Second and First Styles.

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Style Features
In the Julio-Claudian phase, a textile-like quality dominates and seem to be the connection of all the elements on the wall, the colors became warm once again, and they are used to improve the capability of depicting scenes drawn from mythology, panels were also used featuring floral designs on the walls. One of the largest advancements seen in this painting is the improvement of still life with intense space and light. Shading played a key role in Roman still life. This style never truly gained relevance until the 17th and the 18th centuries with the Dutch and English decorations.

Image source: https://search.creativecommons.org/photos/ee270f2f-9baf-43bc-8c1e-d57beb91888b by Following Hadrian

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Scenes of heroes and mythological figures were painted with warmer colors and elements and accessories were painted in yellow gold. In the Flavian period, as in the last years of the city of Pompeii, real scenes and great landscapes appeared. Other developments included the depiction of backgrounds, and the introduction of a style consisting of arabesques on a white ground, as in Nero’s Domus Aurea in Rome.

Image source: https://search.creativecommons.org/photos/7ebea877-8d39-4816-8f69-b4fd8991d082 by Jennifer Mei

Image source: https://search.creativecommons.org/photos/56d9d2f2-c72b-4a5d-9887-4e3768cca3b7 by Following Hadrian
Representative examples
An important example of the Fourth Style is the Ixion Room in the House of the Vettii in Pompeii. One of the main contributions that distinguish the Fourth Style from the other ones is the growing importance given to still life with great spaces and lights. Shading was a key factor in Roman still life. This style was never considered important even if in 17th and 18th centuries will be considered important with the contribution of the Dutch and English scholars.

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Info source:
http://www.art-and-archaeology.com/roman/painting.html
http://www.pompeiin.com/en/Painting_styles.html
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