Partition alert message when using Boot Camp Assistant: "The startup disk cannot be partitioned or restored to a single partition."
You have to delete the Data partition. disk1 (Macintosh SSD) is no real partition/disk but a virtual volume residing in disk0s2 but containing your system.


Backup the content of Data
Open Disk Utility
Choose the superior disk and hit "Partition":


Click on the Data partition and hit the - button:


Click Apply

The Data partition will be removed, the Recovery HD moved to the end of the disk and Macintosh SSD will be expanded:

diskutil list #before
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *68.7 GB    disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage System                  51.2 GB    disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:                  Apple_HFS Data                    16.5 GB    disk0s4
/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS System                 +50.9 GB    disk1
                                 Logical Volume on disk0s2
                                 4AACDE41-E3A1-444A-A5F6-C1491F46149E
                                 Unencrypted
diskutil list #after
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *68.7 GB    disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage System                  67.9 GB    disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh SSD          +67.5 GB    disk1
                                 Logical Volume on disk0s2
                                 4AACDE41-E3A1-444A-A5F6-C1491F46149E
                                 Unencrypted