This implies that your brain could handle 4 arms. Chances are they would just be dead limbs.

Biological systems have an incredible plasticity. That same Michael Levin once used his discoveries to make a tadpole with an eye somewhere near the gut. The eye formed correctly and grew a nerve that connected to the spinal cord. The tadpole was able to see out of that eye. It's definitely a non-stock configuration yet it worked. I'm sure it's much more complicated than having 4 arms.

The problem isn't (just) the brain power, the shoulder joint is also a necessary component of the full range of motion and degrees of freedom your arm has. You can't just slot a ball and socket somewhere into the side of your ribs (and all the supporting musculature) and have it work the same way.

The just in time for Halloween scenario is they grow back but with Alien Hand Syndrome.

So they're constantly giving you wedgies, making rude gestures to loved ones, writing bad checks, and trying to choke you out.

Almost certainly wouldn't be an issue. There's a huge amount of plasticity in embodiment. Think about how easily we can control videogame avatars of many physical forms, or adapt ourselves to a musical instrument, or even learn 'pilot' an artificial limb.

Obviously we can grow an extra brain that will operate the extra limbs.

Conjoined twins Abby and Brittany Hensel look fairly normal apart from the two heads, but are really one-armed amputees each (but they were born with four arms and two heads): https://www.youtube.com/watch?v=VJN2sauvEb0

Our brains can learn to handle new limbs - experiments with sensors that detect nerve impulses and control robotic arms have proven that.

Our brains can learn to handle new senses - experiments with sonar or infrared being sent to panels of pins that push on someone's arm have proven that.

I once spend a ( drunken ) evening discussing with a neurosurgeon about the possibility to add a prosthetic tail to the human body.

The base structures to control a tail are present in our brain, sure we lost it a few million years ago but still a tail is much more realistic than e.g a second pair of arms.

no more need for poles when tightrope walking!

So I wonder how much brain power (if any significant) is being taken up by the uterus during pregnancy.

Well, I don’t know if it’s the brain power or the gray matter pruning that goes on, but my wife was an absolute space cadet for a while during pregnancy and quite a while after. She’d forget almost everything.

I've read literature that the hormone fluctuations resulting from pregnancy, and from parenting regardless of sex actually trigger the destruction of particular parts of the brain in humans.

This is a long shot, but I've always wondered if it might be genetically related to the part of the brain responsible for "abandon one of the babies to escape predator" type reasoning that you see in animals with litters. Something that would have been selected out of human evolution fairly quickly.

Not really. You would need some training but as long as the limbs nerves are connected you can manipulate them.

I don’t think that’s necessarily true. We have mirrored nerve bundles dedicated to limbs with specific junctioning in the CNS. I don’t think it’s just brain plasticity at play, it’s fundamental wiring structure from limb to brain as well.

I think that's wildly over-optimistic: We have plenty of problems with existing limbs when it comes to fixing nerve-damage and then re-training everything. That's also with the advantage of decades of active prior use and in places/amounts that are supported by ten millions years of evolved physiological prep.

Male monkeys of some species are born colorblind. Researchers used viral gene therapy to add the other rhodopsin shapes for the other colors and they learned to process the new info in less than two weeks. Brains are amazingly plastic, but they are finite, something else would maybe have to go.

Babies do it.

Plus all your shirts would need to be tailored.