Linux

I'd go with XFCE

Its fairly difficult to find "up-to-date" performance / RAM comparisons of Linux Desktop environments, but here's a decent one from 2019 comparing memory usage of different Ubuntu flavors.

The most surprising thing is that despite KDE Plasma's reputation as being more ram-hungry, it actually used less ram than XFCE, meaning its developers have been making performance a focus.

Xfce

Does not answer your question, and someone already mentioned it in a thread, but don't forget zram when only 4GBs are available.

I recently bought netbook on AMD c50 for 20$ and firstly, i bought some ram and ssd, luckily ddr3 is very cheap, one or two 8gb sodimm modules and 256gb ssd, or in my case 360gb because price was the same when i ordered them, 360gb was even slightly cheaper, so what i was trying to say, this small cheap upgrade will make a world of difference, and when they'll arrive I'm planning to install "tumbleweed kde" , whole cost of upgrade is 8$ for one module of 8gb ddr3 sodimm, and 17$ for 360gb ssd, 256gb price was the same as i said before

Ironically, ChromeOS Flex would run smooth on those specs, since it does so on my dogshit Samsung Chromebook 3 with shittier specs.

river or sway

I would go mx linux fluxbox

PSA no matter how light your distro, any modern app or webpage will use all that power

LXQt, XFCE, Maté, TDE. Any of them will do. Which you choose depends on personal preference and how large an ecosystem you want—LXQt has only a few basic applications, TDE has pretty much everything that was in KDE3, the others are somewhere in between.

You could try Niri. I have tested it with a ~10 year old notebook with a 1st gen Core i5 cpu.

But, even newest Gnome runs smooth on this machine.

That's a reasonable machine. You probably could use anything but if you want lighter weight you could use Xfce4. If it is a laptop you could use stock gnome with some swap as a backup to prevent OOM

Is the A6 from 2017/18? Should be fine with anything. My wife's laptop is from 2010/11. I tried all the DEs because of the lightness claims, I found GNOME worked the best, and it is super peppy running NixOS.

I asked online why GNOME would perform better than what is assumed a lighter DE, and a comouter dude says GNOME goes and gets everything it needs and caches it when you launch something so retrieval is faster in the app, KDE loads stuff on demand as it is asked for so a alow CPU and HDD hinderes KDE for me.

if you can afford it, by 4 more gigs of RAM

honestly they are all pretty good at this point. start with the default ur distro supports. if that isn't to your taste try kde/plasma, gnome or lxde

A window manager like i3 or Openbox. If you are curious what that's like, then try out Bunsenlab Linux. (XFWM4 is also a great choice, but it requires some know how to properly rip out the rest of Xfce, like the relatively heavy desktop and the panel)

Xfce, LXQt or just install JWM and enjoy the 30 Mb idle RAM usage

KDE plasma. From my experience it uses less resources than lxqt and xfce and works out of the box while lxqt and xfce required extra work to get wifi, screen brightness controls and audio working. I can have 10+ tabs in a chromium based browser open without lag on an old laptop with 2GB ram and 1.33 - 1.83GHz 4 core intel atom from 10 years ago.

If the PC has an SSD, install anything you want, the PC will handle it fine.

Moksha Heck, just install Bodhi Linux 7, your choice between Ubuntu based or Debian based.

If you are still using X, get Fluxbox, very lightweight, requires some config, but that is fairly easy.

Moksha Desktop environment Bodhi Linux

I have a thumb drive with Mint Mate installed on it and it runs fine on a 4gb i5 - 3rd gen.

LXQt, XFCE Or a window manager, they’re all lightweight.

Technically not a DE, but I like plain openbox.