[ponkotsu]

Im trying to figure out the best way to do this. So tell me if this works?

4 bay nas, 2 hdds from the old htpc, but not backed up in the nas, the backups of these are in the other htpc.
2 new drives in the nas, 1 for new content the other a backup of it.
The HTPC will use both the local drives it has now & the new drive on the nas.
The laptop (replacing the old htpc) will play everything from the nas.

I think that will work, however im not sure i can use a centralised MySQL library in Kodi as the source will be different on 2 machines..maybe it will let me though?

Assuming you're using Windows for the HTPC's. Also assuming you currently have the 2 HDDs in each HTPC assigned the same drive letters on each HTPC (for example, HDD1 is L: and HDD2 is M: on both).

That will work, with caveats.

When you put the 2 old HDDs into the NAS, map them (in Windows) as the same drive letters as before on the old HTPC. That way, your old HTPC will still be looking at drive L: for media that was there before. Likewise with drive M:.

When you add the 2 new HDDs into the NAS, share one of them out to the HTPCs. Again, map this share using the same drive letters on both HTPCs (say, drive N: on both).

That should theoretically keep both HTPCs using the same path for the same media. Convert your current standalone library (SQLite I think?) to a central MySQL one, and point both HTPCs to that MySQL instance so that they both use the same library in Kodi.

But I don't use Windows so take that all as theoretical rambling.

Check with the manufacturer of the NAS you want to buy to see if it can install MySql and run it from the NAS.
Also check to see if the drives you intend to use in the NAS are compatible with that NAS.

Also pay attention to these points.
You can usually run MySQL on most of the NAS units out there larger than 2 bays, but research first so you don't waste money on one that doesn't.
And NAS units can be finicky about HDDs. The manufacturer should have a list of compatible HDDs for their NAS on their website.