Will a EXT4 Samba Share be readable by Windows/Mac OS X users?

The underlying filesystem has little to no impact on SAMBA, and yes, I can tell you definitively, that EXT4 samba shares to other OSes, which do not themselves support EXT4.

SAMBA presents its own filesystem semantics which overlay the physical filesystem, so your clients will not be able to tell at all what the underlying FS is.

just make sure that the filesystem permissions on your shared directories and their contents allow the remote user the access they require, and do the same in SAMBA.

Yes. SMB is a network protocol, and SMB clients don't care how the SMB server is storing the data.