I just went through the various 15" configuration options and it appears that ONLY the 4TB model has a 3-4 week delay. Not sure what that tells us. My most optimistic scenario is that Apple is dropping the 4TB option on the 15" and offering it on the 16" instead.
The 3-4 week delay suggests Apple is no longer manufacturing 15-inch MacBook Pro. The assembly lines are in the process of conversion. The only stock that exists now is previously manufactured logic boards. 4TB chips are extremely price sensitive, so it makes sense Apple isn't keeping it in stock.
The delay was previously 5-6 weeks, then 4-5 weeks. Today, it's 3-4 weeks.
It's extremely unlikely Apple is facing any delays in getting 4TB flash. Rather, Apple is not stocking 4TB logic boards and are telling customers to wait 3-4 weeks.