Mongo 4.9+ cannot start on M1 laptops:

% /tmp/mms-automation/test/versions/mongodb-macos-x86_64-4.9.0-rc0/bin/mongod
zsh: illegal hardware instruction  /tmp/mms-automation/test/versions/mongodb-macos-x86_64-4.9.0-rc0/bin/mongod

acm tracked down that this is because the ISA targeting for the server build was upgraded in 4.9 to sandybridge, which uses AVX instructions that are not handled by Rosetta:
https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment

Rosetta translates all x86_64 instructions, but it doesn't support the execution of some newer instruction sets and processor features, such as AVX, AVX2, and AVX512 vector instructions.

He recommended special case downgrading the macOS target ISA to something that doesn’t generate AVX until we have a native Apple Silicon build.