You'd be hard pressed to find an old laptop with 32GB of RAM, and a laptop doesn't provide the same operational conditions experienced by a cluster of 4 computers communicating over a network connection.

Scaling horizontally presents entirely new classes of challenges that aren't experienced while scaling vertically.

ThinkPad W530 fits the bill quite well. Broken screen puts it at around $100-150, $300 gets you fully working one. Less tyhan 4 CM4 modules, and what about Turing Pi 2 case? power supply?

>Scaling horizontally presents entirely new classes of challenges that aren't experienced while scaling vertically.

Pay more for challenges? If you really want to fight with bad scaling just run a VM.

Couldn't you buy four 10 year old laptops?

Anyone can buy a brand new raspberry pi 4 with 4GB of RAM for around 50€, which fits on the palm of your hand. You can even get them delivered to your doorstep with a few mouse clicks.

The same can't really be said for 10 year old laptops.

Yes, for the former you have to walk across the room and dig in the closet, optionally ask friends.