I'm starting to use my Raspberry Pi 4, connected to 1 SSD & 1 HDD, as Samba server & Resilio Sync. Is this considered NAS?

Does the RPI4 connected to SSD and HDD simultaneously have enough power supply? You might want to pay extra attention here though...

I try with RPI3B+ and it already start doing weird stuff

My synology DS414 drives indicated "Failing" on 2 of the 4 drives.

1. do I need to replace them immediately ?
2. can I replace the fail drive from WesternD to Seagate ?

If you don't want to risk losing the data, better replace immediately. How you configure for the 4 drive? Using RAID2?

Brand doesn't matter, can use any. As long the drive is not SMR.

i use SHR. i will replace 1 at a time. my current installation is 4x3TB. i will replace with 1x8TB first right

i use SHR. i will replace 1 at a time. my current installation is 4x3TB. i will replace with 1x8TB first right

Taiwan-based NAS maker Synology has warned customers that the StealthWorker botnet is targeting their network-attached storage devices in ongoing brute-force attacks that lead to ransomware infections.

The NAS maker urges all system admins and customers to change weak administrative credentials on their systems, to enable account protection and auto block, and to set up multi-factor authentication where possible.

Synology rarely issues security advisories warning of active attacks against its customers. The last alert regarding ransomware infections following successful large-scale brute-force attacks was published in July 2019.

The company advised users to go through the following checklist to defend their NAS devices against attacks:
-Use a complex and strong password, and Apply password strength rules to all users.
-Create a new account in the administrator group and disable the system default "admin" account.
-Enable Auto Block in Control Panel to block IP addresses with too many failed login attempts.
-Run Security Advisor to make sure there is no weak password in the system.

"To ensure the security of your Synology NAS, we strongly recommend you enable Firewall in Control Panel and only allow public ports for services when necessary, and enable 2-step verification to prevent unauthorized login attempts," the company added.

"You may also want to enable Snapshot to keep your NAS immune to encryption-based ransomware."

Synology warns of malware infecting NAS devices with ransomware
If you still using admin account with the name "admin", better create new admin account and disable this admin account.
Use strong password. Use 2FA. Enable auto block. Change from default port number and don't port forward unnecessary port.

..should I temporarily disable QuickConnect 

If you didn't encounter any login attempt recently then should be no need.
This is mostly on older DSM when initialized that didn't disable "admin" account.

I run into a very complicated problem

Nas 1
[4TB 4TB 8TB 8TB)

Nas 2
[3TB 3TB 3TB 3TB)

my Nas 2 has disk 1 and disk 4 failing. So I buy 2 x 8TB. plug the 8TB into Nas 1, remove the disk 1 from Nas 2 and plug into disk 1 of Nas 2. then I got system partition error on all drives on Nas 2. what hav I done wrong ?

Do you wait until first 8TB finish rebuild before you insert second 8TB?

Edit: Oh your problem is with NAS 2. Maybe when you put in the HDD into NAS 2, since the HDD still have data from NAS 1, it confuse NAS 2 as now it see 2 different data set.

can i just format the NAS HDD to NTFS file format using windows 10?

I manage to clone the HDD and insert into NAS 2. it ask me to confirm that I want to overwrite the data from another NAS (even though I already clone it). so now I just kept my finger cross. but truely life is really painful when 2 HDD is failing at the same time.

i am now rebuilding disk 1. I kept getting bad sector notice on disk 4. and the rebuilding process will take me 28 hours.

I run into a very complicated problem

Nas 1
[4TB 4TB 8TB 8TB)

Nas 2
[3TB 3TB 3TB 3TB)

my Nas 2 has disk 1 and disk 4 failing. So I buy 2 x 8TB. plug the 8TB into Nas 1, remove the disk 1 from Nas 2 and plug into disk 1 of Nas 2. then I got system partition error on all drives on Nas 2. what hav I done wrong ?