Raspberry Pi 3 was a Cortex-A53 back ported to 40nm outright.
Raspberry Pi 4 is on 28nm. (for comparison, Apple A8 and Snapdragon 810 were on 20nm already, in 2014-15)
It runs with a Cortex-A72 at a low clock (1.5GHz, phones even are at twice that clock nowadays) and quite high power consumption because of the process node.
The memory interface is narrow (32-bit bus, phones ship with a 64-bit bus and laptops/desktops with a 128b one) at a low data rate (LPDDR4-3200).
In addition to that, the CPU can only use 5GB/sec of it, with the remainder being reserved for the GPU only.
Those were some of the sacrifices needed to reach this price point. (and why it isn't representative at all of the performance of higher-end ARMs)
At the same time Intel was heavily subsidizing their x86 mobile SoCs to tablet manufacturers and for a while it was actually quite competitive price wise but it ultimately went bothered.
I have a _very old_ PN40 from Asus -- this is a full PC with an intel Celeron, a SATA SSD and about 8GB of RAM that idles at 1.7W (serving websites via GB Ethernet) as measured _at the wall_. This is just standard PC Linux distro with zero customization (other than `powertop --auto-tune`).
For comparison, the latest RPI4B without the SSD and with 1/8 RAM idles at 3.5ishW on the wall. Even the older RPI3 I could never get below 2W, and that is without any peripherals (no display, no Wifi, no eth, min USB) and significant tuning.
On the other hand the PN40 + components cost significantly more (probably close to 10x), and the CPU performance itself is not that good these days.
https://raspi.tv/2019/how-much-power-does-the-pi4b-use-power...
- The Odroid H2+ x86 board claims about ~4W idle power: https://www.hardkernel.com/shop/odroid-h2plus
- This measurement of idle power on the Raspberry Pi 4 says it uses ~3.4W: https://www.tomshardware.com/reviews/raspberry-pi-4
in the past as a relatively cheap x86 CPU. ($9.62 per unit)
However, it's a 400MHz 486 with some backports (1st generation Pentium, no MMX even) and broken LOCK prefix so that software specifically needed to be recompiled for it.
And at 2.2 W too, it never ended up succeeding anywhere, with no successor.
The right era pentium had the same LOCK prefix issue (we used to call it the F00F bug)
Edit: Thanks for link; I must have anchored on the instruction set and ignored that it was using the 486 pipeline.
"The new x86 CPU uses an old-school 486 scalar pipeline and the original Pentium instruction set with some modern enhancements"