I found the header used by the MidiPort library:
http://doc.ddart.net/msdn/header/includ ... tem.h.html
#define MMSYSERR_BASE          0#define WAVERR_BASE            32
#define MIDIERR_BASE           64
#define TIMERR_BASE            96
#define JOYERR_BASE            160
#define MCIERR_BASE            256
#define MIXERR_BASE            1024
/* general error return values */
#define MMSYSERR_NOERROR      0                    /* no error */
#define MMSYSERR_ERROR        (MMSYSERR_BASE + 1)  /* unspecified error */
#define MMSYSERR_BADDEVICEID  (MMSYSERR_BASE + 2)  /* device ID out of range */
#define MMSYSERR_NOTENABLED   (MMSYSERR_BASE + 3)  /* driver failed enable */
#define MMSYSERR_ALLOCATED    (MMSYSERR_BASE + 4)  /* device already allocated */#define MMSYSERR_INVALHANDLE  (MMSYSERR_BASE + 5)  /* device handle is invalid */
#define MMSYSERR_NODRIVER     (MMSYSERR_BASE + 6)  /* no device driver present */
#define MMSYSERR_NOMEM        (MMSYSERR_BASE + 7)  /* memory allocation error */
#define MMSYSERR_NOTSUPPORTED (MMSYSERR_BASE + 8)  /* function isn't supported */
#define MMSYSERR_BADERRNUM    (MMSYSERR_BASE + 9)  /* error value out of range */
#define MMSYSERR_INVALFLAG    (MMSYSERR_BASE + 10) /* invalid flag passed */
#define MMSYSERR_INVALPARAM   (MMSYSERR_BASE + 11) /* invalid parameter passed */
This is what I have found in the mmsystem.h file, used by MidiPort. Not 100% sure, but the error returned corresponds to this list, so if you get error code 4, then the error is 
device already allocated.
But I don+t know what it means. Maybe some MIDI connection is opened.