You'll have to be a lot more specific than "ARM" - Most newer ARM systems are little endian in practical operation, and ARM has been "flexible endian" (you can switch between big and little endian - SCTLR has the relevant bits to control the accesses on most recent ARM ISAs) for some long while now.

I just ran it on my M2 mac and got 04. Don't compilers typically take endianness into account for things like this anyway?

No, compilers don’t take endianness into account. (especially not C)

You need to use a bit mask in order to make this code endian-independent rather than a pointer alias. Like (uint8_t)(int & 0xFF), or something like that.

Why would it do that? You're asking for a raw memory address value.

IIRC several currently used architectures (e.g. ARM, Power, z/Architecture, RISC-V) can all run in big-endian mode.

And in the embedded space I think PowerPC and MIPS are still around.