Version of
Microsoft DOS released in 1988. Dos 4 had serveral improvements over the previous version of DOS, version 3.3. Among these were support for drives larger than 32 megabytes, an integrated
disk cache, and a graphical file management shell.
Unfortunately, it also had a lot of bugs. The RAM Disk was severely flawed, and would often lose data, making it nearly useless. A poor buffer setting would cause severe disk corruption when dealing with files larger than 32 megabytes. Even such basic features as getting a directory listing were buggy, with the OS randomly causing the dir command to beep whenever it gave a listing. These bugs and more gave DOS a black eye, and people were not happy.
Customer complaints began soon after the OS was released. Many of the bugs caused severe data loss and corruption, making the OS unacceptable for use in nearly any environment. Most people purchasing new computers in this era would demand that DOS 3.3 be installed. That previous version was considered rock-solid, and people would often pay a higher price for the "privilege" of a lower version.
Microsoft attempted to solve the problem by releasing DOS 4.01, but the damage was already done. DOS 4.01 still had a considerable number of bugs, and was tainted by the bad reputation of its predecessor. Even if MS had fixed the problems, most people wouldn't buy any revision of DOS 4; it wasn't worth the risk.
If it hadn't been so buggy, DOS 4 would have been remembered as a significant milestone in the history of Microsoft. It was the last OS to include a copy of their linker, the last version to include gw-basic, and the last version of the pre-windows era. Due to its bugginess, however, it's instead remembered much along the same lines as the IBM PCjr; a good idea, but severe flaws killed the imlementation of those ideas.
Mad props go off to Frater 219 for the PCjr analogy. It's a much better analogy than my former example.