Thread 1.4 is a crucial update for Matter, as it fixes a number of problems and now Thread will finally be able to join, by default, to existing Thread networks, expanding your current network instead of creating a new one.
Apple just updated its Border Router to Thread 1.4, and Google, according to the Matter Alpha, is testing the update internally.
However, I haven’t seen any news about SmartThings yet.
That was already possible with Thread 1.3 and we know some vendors like ST decided to manage their own isolated network and never used the mechanisms in place to join others or let other TBRs join. I’m not sure if the spec “forces” vendors to join a existing Thread network, apparently it didn’t in Thread 1.3 so…
What Thread 1.4 makes easier is connecting to a given Thread network even if you don’t have the credentials (that’s a bit spooky, reminds me of WPS except this time you have to authenticate in the vendors app and the codes are valid for one time only so feels more secure). The whitepaper of Thread 1.4 describes the mechanism as something to be used exceptionally:
The procedure described below only needs to be executed in exceptional cases where the Thread network credentials are unavailable to the user performing the onboarding of a new Thread device.
Which, if you are managing your Matter/Thread networks from the same device shouldn’t happen in practice as the Thread keys should be in the local key store.
Mark Benson, head of SmartThings US, said Samsung’s border routers will be on 1.4 “later this year.”
I’ve been able to manually add a third party TBR to a SmartThings thread network. What I’d really like is to be able to add an ST hub to a SmartThings thread network as that is basically the only useful thing a hub group achieves for me.
That doesn’t need 1.4 though it appears 1.4 might effectively make that the default behaviour. The worry will then be that the non-default behaviour is also available as users might want multiple thread networks.
Yep, you can add third party TBRs to SmartThings as long as you have the ST network credentials, which can be easily extracted with a Nanoleaf light and stored in the phone with the Home Assistant companion app.
What SmartThings never wanted is being able to join other Thread networks, that was on purpose for stability and reliability reasons, there’s an interview in a matter blog I can’t find right now.
Unless Thread 1.4 actually forces the behaviour to manufacturers, things won’t change a bit.
I think all I needed was the network key which is available in the ST app.
That depends on the kind of TBR, if it’s a OpenThread is enough with the key, if it’s, let’s say, a Google Home or other platforms TBR, it only reads the credentials through the Android API and you can only write that with the HA app.
Yep, exactly! I wrote a blog post about it: How to unify the Thread networks from SmartThings and Home Assistant? - PontoBYTE 
In fact, this is a very requested post.
However, SmartThings won’t let me connect to another network. In my case, I can’t connect to Apple’s network, so my SmartThings is creating a different network.
Think of all the nice fabric you’re collecting!
No seriously this whole situation is a shambles, I now have four thread networks.
