I have bad news.

The hammerhead may not be able to get SFOS 4.0.1. The reason its old kernel version which is 3.4.
On the release note of SFOS 4.0.1 Koli it says: "The lowest supported kernel version of Sailfish 4 in the remaining Sailfish OS devices is 3.10."

I've built a 4.0.1 image for the hammerhead but it doesn't boot (stuck on the boot logo) as /system is not mounted. If we don't find a workaround, the latest SFOS version for this great device could be 3.4.0.24.