How to add or join that node on proxmox ??

That is not a node, that is a pool, see Datacenter -> Pools..

Also the 4.x release is EOL since almost two years, and 4.1 even longer (latest release was 4.4 of that series).

You're missing lots of bug (and thus also security) fixes, especially the whole meltdown/spectre CPU stuff.

Please try to update as soon as possible!
Setup valid packages repos: https://pve.proxmox.com/wiki/Package_Repositories
Upgrade 4 to 5 https://pve.proxmox.com/wiki/Upgrade_from_4.x_to_5.0
Upgrade 5 to 6 https://pve.proxmox.com/wiki/Upgrade_from_5.x_to_6.0
 
Your life will be easier, trust me.

Depends, 4.4 to 5.x is a very painless and easy-straight-forward upgrade especially if one follows the Upgrade documentation step by step in the correct order.
Proxmox VE 5.4 to 6.x isn't to hard either, the only thing one needs to look out is for the corosync 2 to 3 upgrade step, but the documentation and the pve5to6 are quite battle tested and work out.

But yes, backing up all and doing a fresh installation has also it's merits and works out good, just ensure you tested the backzup restore first to ensure it works (this should be always done for backups once in a while, as else they can be worthless and one notices only when it's to late).
 
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