As far as I understand, the 4 digit passcode is combined (in some fashion) with a key stored in secure read only memory (e.g. secure enclave chip or similar), where it is directly embedded into silicon wiring to help prevent unauthorized reads.

But no matter how strong or multi-layered or complicated the security is, wouldn't it still be possible to read the key directly from the silicon wiring of the secure chip or ROM, using some electron microscopy technique or similar? If so, surely the FBI could develop the technology for this, without asking Apple for help.

Yes, it is possible.  However, that runs the risk of destroying the device without getting the data off first, which is undesirable.  It also does not achieve the political goals of forcing Apple to assist in decrypting the device, paving the way with precedent for the flurry of future requests of this sort to come, some of which are certain to have less favorable facts and thus are not as suitable as test cases.   