4.01 DOS Operating System Microsoft April 1989 MS-DOS 4.01 (360K Disk 1) MS-DOS 4.01 (360K Disk 2) MS-DOS 4.01 (360K Disk 3) MS-DOS 4.01 (360K Disk 4) MS-DOS 4.01 (360K Disk 5) MS-DOS 4.01 (360K Disk 6)