Those new "shingled" Hard Drives are 8TB and pretty cheap. However, the hard drives have severe reductions in speed and are really only for archival purposes. IIRC, there's a path for 10TB, 16TB and beyond if you're willing to put up with the weaknesses of shingled hard drives.
Granted, SSDs are moving from MLC(2-bits per cell) to TLC (3-bits per cell) which reduce speed significantly without actually selling any "more" hardware. So both sides are "cheating" extra capacity out of the same hardware.
If you want to keep the same high-quality specs that the 4tb hard drives have (speed, reliability, etc. etc.), I don't think there's a valid upgrade path at the moment. There's some research to push the capacities beyond that while retaining the speed of current hard drives, but they're not ready for commercial use yet.
But the same is true for SSDs. 16nm MLC Flash might turn to 10nm or 8nm as process technology improves... but that's maybe a 2x to 4x improvment in capacity. TLC gives another 50% boost.
HAMR may also manifest at some point soon and push HDD densities way up while getting rid of the problems associated with SMR. HAMR will likely be expensive just like Helium is but it will certainly unlock a lot of super high density materials when it comes about (which may be in the next several years).
With the tech we're talking about, there are definitely limits per inch. They'll be able to fit a petabyte in a 2.5" SSD drive case before the SSD sunsets, at a cost of $300. At that time, your 100tb SSD might be $50 to $75.
After SSD, what technology will take us to 100pb drives at $300? Is that even possible, while keeping the small size of the drive? No idea. It's going to require some amazing breakthroughs.
DNA storage[1]? It will be very interesting to watch computers become more "organic" over time. DNA for storage, processors that are correct "most of the time", deeper and more broad neural networks, etc.
Is there a point at which they simply cannot get cheaper?