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system4

System 4 was introduced with the Macintosh SE and Macintosh II.  System 4.0's major claim to fame would be the fact that it was the first System to exclude a model - the Mac 128k. It did have some fixes to accommodate AppleShare, like a new Chooser and a new Control Panel with an AppleTalk section. System 4.0 was recommended for all Macs with 512k of RAM and higher

system4b

System 4.1 

  • Multiple monitor support.
  • Added support for disk drives larger then 32MB.
  • This System also excluded a model, the Mac 512k, since you needed 1 MB of RAM.
System 4.2
which was also termed System Software 5.0 (not to be confused with System 5, which never really existed), incorporated Finder 6.0. Apple started to use "System Software" versions with this release. Users were confused in tracking the System and Finder versions separately, and thus a "packaged" name for the OS was used.

Improvements:
Implemented MultiFinder - which could support multiple programs, though it was not true multi-tasking.
A customizable Control Panel program.
The “About Finder” dialog box not included memory usage. the mountain scene previously there was re-located into an “Easter Egg
On Macintoshs that supported color, the Color Menu made its debut.
Progress bars were added to dialog boxes and a cancel button.
Several new utilities, including Backgrounder, DA Handler, Easy Access, Print Monitor as well as background printing was added.

System 4.3
Was a service upgrade - bug fixes and some updated printer drivers.