Often these scammers use services that provide temporary numbers or even Telegram accounts, and after a period of time they sell them or the service provider sells them, and a third party may buy them. Therefore, this third party does not necessarily have to be a scammer, but it is better not to deal with any account on Telegram without using escrow.
Back to your story, did they pay you the remaining $4,000, or did the project stop, or what?
In the story - the client disappeared and could not be contacted after the initial payment, possibly subsequent payments were not made. But this is an old story that was then pumped back up by the OP under the pretext of warning others about the potential for fake people controlling his client's telegram account. If the OP is honest and has no other motive - then it would be logical for him to send the bitcoin back to his client's address without shouting to find out where the client is, especially since he knows his client is a large crypto owner.
Alarmed and wary, I took a step back to assess the situation. The disappearance of my original client, coupled with the change in name, raised concerns about the legitimacy of the individual on the other end of the screen. It begged the question: Was this the same client, or had their account fallen into the wrong hands? Since this client held a large amount of crypto, could he have lost his phone and more? Was he kidnapped?