Interesting. Do common server (or even consumer) workloads contain enough CPU "dead time" to make 4 threads per core a benefit?
There's been >2 hyperthreads in other server CPUs before. IBM and Sun/Oracle now or in the recent past have sold systems with 4-8 way threading. Of course, these would be niche systems even without lots of threads per core, so one's definition of a "common" workload would need to be adjusted accordingly. My experience was that they worked really well for big databases and Java app servers, less well for other uses.
When you'll have 4 cores per cpu, there will be usecase fot it. Generally IO is catching up with cpu performance, so it makes sense to do this.
"dead time" occurs when a thread is stalled. The most common reason is a cache miss. So if the running applications have sufficient cache misses, additional threads may help - provided that the applications are multithreaded.