Released along with the 800 in 1979, the 400 was the low-end model of the two.
The only 8-bit Atari with a membrane keyboard rather than a full-stroke
keyboard.  One of the few 8-bit Ataris lacking a composite monitor port.
Originally released with just 8K RAM, but most were sold with 16K RAM.

Atari sold the 48K RAM Expansion Kit for the 400, which required a little
soldering, to dealers only.

Most Atari 400 machines include a standard 6502 microprocessor, but late-
production units use a revised CPU Board that features Atari's SALLY 6502.

On the 400, joystick controller port #4 is the only port that supports a light
pen or light gun.

Features unique to the 400/800 models:
  - Four controller (joystick) ports
  - Internal speaker for keyclicks and system buzzer
  - Memo Pad mode
  - +12 volts on pin 12 of the SIO port

Boot options:
   Memo Pad
      - Turn on computer with no cartridge inserted and no powered disk drive.
   Cartridge
      - Turn on computer with cartridge inserted.
   Cassette
      1. Hold down [START] while turning on the computer.
         (System buzzer sounds.)
      2. Press [PLAY] on the program recorder.
      3. Press [RETURN] to load and run cassette program.
   Disk
      - Turn on computer with disk inserted in powered disk drive.

Versions of the Atari 400:

  o  NTSC (North America) version
     - TV Channel switch: (2 - 3)
     - CTIA (early production) or GTIA (most)

  o  PAL (Europe) version
     - TV Channel switch (channels vary by country)
     - GTIA

Rare variation of the 400:
  o  At least some of the few Atari 400 units (PAL) sold by Atari in France
have been reported to include a built-in peritel cable.  PICTURES, ANYONE???
http://www.atariage.com/forums/index.php?showtopic=62346&st=25

Atari marketing used the trademark, The Basic Computer, as an alternative name
for the 400 from 1981-1982.

The 400 was made in the USA (early production) and Hong Kong (later
production).

Production of the 400 ended in May 1983.