Recently, as I built VirtualBox 6.0.8 from source in my Debian -based AMD64 Metztli Reiser4 development environment, I came across pre- Linux kernel 4.14.x virtual machines which had been formatted in Software Format Release Number (SFRN) 4.0.1. It is important to remember reiser4 in the context of Linux kernel 4.14, because:
"In accordance with Reiser4 development model we announce a new software framework release 4.0.2 of Reiser4 kernel module and Reiser4progs. So, before the next mount session please don't forget to check your old reiser4 partitions by new fsck to update things (not urgently)."
Reiser4 for Linux-4.14, Reiser4progs-1.2.0, Libaal-1.0.7, Format 4.0.2

My Metztli Reiser4 for AMD64 current development environment is running reiser4 -enhanced Linux 5.0.15-1 hacked around a Debian kernel packaging for 5.0.13-1 experimental wrapper from master branch:

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yields:
Linux huitzilopochtli 5.0.0-1+reiser4.0.2-amd64 #1 SMP Debian 5.0.15-1+reiser4.0.2 (2019-05-11) x86_64 GNU/Linux

Side note: In an effort to diagnose sporadic build-time reiser4 -patched kernel hangs which crept into kernels 4.15.x and above, I have based its build in solid Debian packaging for Linux kernel 4.14.x debian/config/config (attached below) which directives customize the final debian/build/build_amd64_none_amd64/.config that builds my reiser4 hacks of Debian kernel packages for Metztli Reiser4 installer UDEBs, as well as userland DEBs.

To build VirtualBox 6.0.8 on Metztli Reisr4 development environment for AMD64 I fulfilled the following dependencies:

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dmesg : found disk format 4.0.1

Once I had VirtualBox 6.0.8 successfully built, I enabled vboxdrv and vboxnetflt modules, set relevant path to its bin directory, fired up executable VirtualBox type 2 hypervisor from my shell in Terminology 1.4.99, and selected at random an approximately 400Gb virtual machine formatted in old gzip -based transparent compression.

Once inside the virtual machine I upgraded the Metztli Reiser4 Debian -based software:

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It was running reiser4 -enhanced Linux kernel 4.12.xy with equally relevant older reiser4progs 1.1.x.y. Accordingly, first thing I did was install reiser4progs 1.2.x.y from Reiser4 SFRN 4.0.2 reiser4progs 1.2.x-y for stretch-backports.

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Subsequently, I installed my current Metztli Reiser4 enhanced Linux kernel from Reiser4 SFRN 4.0.2 Linux 5.0.15-1 for Debian stretch-backports

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Then I powered off the virtual machine. I knew I had to mount it under another recent virtual machine to completely upgrade it to reiser4 SFRN 4.0.2. Not difficult since I often test reiser4 formatted slices in VirtualBox which I then customize for Google Compute Engine (GCE) cloud environments. I chose a reiser4 2TB slice host, Xochiquetzal, to complete the SFRN 4.0.2 upgrade procedure by mounting the target 400Gb under xochiquetzal's /mnt point.

The result is the ~ 6 minutes video teaser above.

Indeed, I also showcased (besides my typing mistakes ) my fork of GNU Parted hack, Reiser4 Parted 3.2-21 for stretch-backports, which I installed as:

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Needless to say, all the reiser4 kernel and utilities I used are automatically installed if you do an Metztli Reiser4 USB/ISO netboot based Debian for AMD64 installation.

References:
Xochiquetzal : Gorgeous Flower -- literally -- a Divinity and Archetype of the Mexicah Concept of Female Beauty.
Вики Одинцова ~ Viki Odintcova
Елена Воронина ~ Elena Voronina
Eлена Ващенко ~ Elena Vashchenko
Евгения Зотова ~ Evgenia Zotova