one disk failed in RAID 5, no data in shared folders, old three disks color changed to RED

Have you tried booting with just the three old drives?  The proper way to replace a bad drive is with the power on, by the way.  If booting with the three orignals works, then add the new one with the power on.  If the NAS complains that it has data on it, you will have to re-format it from the Volume page before it will add it to the array.  It is always a good idea to make sure your backup is up to date before adding a new drive because the re-sync process is drive intensive and if it causes a second, marginal drive to fail, your volume is lost.