I'm afraid that if you want to buy an affordable 4-axis CAM system, you'll have to bite the bullet and leave the MacOS bubble (sorry about that mixed metaphor). But you probably can run a version of Windows on your Mac, using Parallels or whatever. Either that, or just buy an older Windows computer that will come with a Windows OS and probably be cheaper than buying Windows for your Mac. Once you've got something that will run Windows, VisualMill is probably going to be your best option, if you want to make mechanical parts. Unless you get the Windows version of Rhino, RhinoCAM won't work, but Mecsoft's VisualMill is the same program in a standalone version, so you;d just have to import the files into it to generate your toolpaths. We can probably save you enough on VisualMill to pay for a pretty good Windows computer - they aren't all that different from Macs these days, if you can handle the jolt to your identity...