Introduced in time for Christmas of 1979 the Atari 400 was the little brother of the Atari 800. It differed from the 800 only in form, not in function. Both Atari computers had the same processor and chip sets. The 400 had a membrane keyboard (kid safe) where the 800 had a full-travel keyboard and the 400 was limited to one expansion slot and, at least initially, a fixed 16k of RAM.
The Atari 400 computer is a 6502 based machine with custom graphics and sound chips that made it a gamers delight. The Atari line had a fairly wide variety of peripherals available including tape and disk drives, printers, modems and interface units. The 400, like the 800, was compatible with them all.
The Atari 400 also worked with all Atari computer cartridges that could run in its more limited RAM.
A new boxed example of the Atari 400 was recently added to my collection.
It came with boxed Educator and Entertainer packages, also from Atari.
Included with the Educator package is a nice, clean and apparently never used Atari 410 program recorder.
Elsewhere in the other boxes are near-new examples of several Atari game and application packages and paddles and joysticks still in their boxes.