I also started getting 4000 error. On one occasion, the error appeared on the OSD screen while hovering, and another time I have seen it on landing. The VTOL copter part was perfroming apparently without issues, however, after one flight I was not able to read the log file, it was saying that FTM data was missing.
I understand that 4000 could be related to SPI communication, and from what I think, speedybee has the SD caard aand the IMU on SPI.
The Ardupilot Hardware Report had this entry:
0x4000: SPI fail (line 215)
I found another post, Internal error 0x4000 l:215 spi_fail
where a similar problem appeared.
What I do not understand whether this is very serious or not, i.e. whether there is a risk of loosing an IMU (and copter goes down), or whether this is just an occasional drop out of a data frame. CPU load was around 50 % and the average loop frequency was 400 Hz, with worst loop rates around 340.