RAID1 /dev/md4 lost one disk partion /dev/sdb4 after reboot, WHY??

[L]

Hi

I have RAID1 system,  the partition /dev/md4 is in raid1 with
/dev/sda4 /dev/sdb4. In order to check /dev/sdb4 I remove it from
/dev/md4

mdadm /dev/dm4 --fial /dev/sdb4 --remove /dev/sdb4

after checked it, I added it again
mdadm /dev/md4 --add /dev/sdb4

The system did sync md4, this lasted ~ 6 hours. however, after reboot,
/proc/mdstat shows /dev/md4 has one disk /dev/sda4 active.

I have no choice, but add it again. the system do sync once again.
apparent something  is not right.

How to let system hold both partitions /dev/sda4 and /dev/sdb4 for
/dev/md4 after reboot?

thanks

Y