The Acorn A4000 was a computer which was designed to compliment the A30x0 line of machines (A3000, A3010 and A3020), and was quite a departure from that series in many ways.
Unlike these machines' all-in-one approach (where everything was in the keyboard, including the floppy drive et al), the A4000 was more like an IBM clone in its design, with a seperate keyboard that plugged into it instead. (Oddly, the mouse plugged into the keyboard, Mac-style). Essentially, the A4000 was a lower-end A5000, its case being smaller and its specifications similar to the A30x0s.
In traditional Acorn fashion, the operating system was on ROM, now with RISC OS 3 being the default instead of the clunky OS 2. This boasted many advantages over the previous incarnation, such as (finally) changing file copying from a single-tasking CLI operation to a proper GUI command. The days of having to patiently wait for a copy to finish before getting on with one's work were over.
Internally the machine boasted specifications well in advance of older Acorns. A 32-bit ARM250 running at 12Mhz was powerful stuff back in the early 90's, and the massive 2-4Mb of RAM ensured this was a system with potential. As for storage, eat your heart out A3020 - sporting not only the ubiquitous 3.5in floppy disk drive, but an 80Mb hard drive as well meant the A4000 was now the top-of-the-range Acorn system.