'4' captures Diagonal in full creative flow and with a new twist to their sound as they incorporate elements of space rock, kosmische and post rock to bolster their well-established progressive rock roots. That adage of waiting ages for a bus and two or more come along at the same time seems applicable to their recent productivity. Returning only two years after the release of their previous LP ''Arc'', this is a notable break from tradition, having left five and seven year gaps before the previous two. And after much deliberation, the band settled on following in the footsteps of Soft Machine and Led Zeppelin in naming the album after its chronological appearance. The band returned to the studio only a year after the sessions for 'Arc'. The result is an assured work that functions wonderfully as a logical progression from 'Arc' yet at the same time has enough shift in sonics to illuminate a different facet of the band. Opening cut 'Amon' (a working title that ended up sticking) makes perfect sense as a reference to the German experimental rock band Amon Düül II. Its solid guitar riff foundation careens over an ever-evolving time signature, the three-part structure decreasing in intensity until it dissolves in a cloud of bubbling ambient textures. 'Chroma' explores the band's interest in jazz. One of the two instrumental tracks on the LP, it showcases the effervescently emotive sax work of Nicholas Whittaker with an intricately crafted solo that unfurls itself over a fluid 5/4 rhythm. 'Spinning Array' is in many ways the track that acts as most of a connecting bridge to the previous album. The stridently crisp bass and drum syncopation of the opening bars giving way to long unhurried guitar lines and vocals. Vocalist/saxophonist Whittaker incorporates some intriguingly different sounds into proceedings with the use of Chinese wind instrument, the Hulusi, as well as the more traditional recorder. The keyboard presence on this album is less pronounced owing to an instrumental/personnel swap. 'Stellate' is a shining example of this shake-up and demonstrates the band at their heaviest in what emerged as a controlled jam. The woodwinds carry the Middle Eastern-tinged