I have successfully ran DNSMasq, WebFS, qBittorrent, VSFTPD and Syncthing at the same time on a OrangePi Zero with 4 cores and 512MB of RAM. That thing is barely bigger than a postage stamp.
I'll switch to a much more powerful device, because I need more hardware acceleration for video encoding and such, but other than that, the same Pi Zero can still handle the tasks needed.
Took up a lot more space than a stamp though.
I think you can still run modern versions of these software on that hardware, albeit with a bit lower performance due to all added features. Kernel would require a bit more resources, too, possibly.
The biggest problem on the OrangePi Zero was to run SSH at full speed, due to required encryption/decryption on the fly. DNSMasq and VSFTPD was essentially invisible, qBittorrent requiring 85MB of RAM to handle all the state data.
While the applications are tied down with cgroups to prevent pushing each other to the reaper of OOMKiller, none of the applications have died because of insufficient memory, which is telling, in a good way.
It shows that my point still stands. Basic functionality on a software is very cheap, but when you add cryptography, or a couple of compute-intensive features, the old hardware breaks down instantly. It's of course possible to optimize these up to a point, but dedicated cryptography accelerators and other instruction sets are really helpful.
If you run a P4 with modern distributions of linux, you're likely in for a bad time.
But if you take the time to customize a distribution of Linux for the hardware, you're not going to have much of an issue.
The biggest problem by far is the modern web browser is an absolute bloated pig.
https://www.sevarg.net/tag/raspberrypi/ has some hints and tricks.
What universe are you operating in? I can run multiple domains worth of services just fine. Yeah. Compiling at -j4 at the same time can be a PITA. Running the desktop over X with compression over ssh works just fine as well.
The key to outpacing a pentium with a Pi4 is entirely dependent on structuring your workloads correctly. Given a heavily serialized load, a Pentium'll smoke a Pi 4 before it even gets it's britches on. Throw in enough parallelizable workload, however, and the Pi4'll pull ahead promptly.
Have you tried running home assistant from an old Pentium 4?
It sure doesn't feel like it. It was never questioned if a Pentium 4 would make an OK desktop, of course it did. The Raspberry Pi 4,... It feels like it should be completely fine for email, Internet browsing, word processing and pretty much any office work, but it's just to slow. People are even maxing out a Raspberry Pi 3 (or 4) running home assistant... Yet an old DOS machine can run an entire factory production line (or multiple).
We really need to do much better on software.