Just because we're still pushing ahead with Acorn 5, doesn't mean we have forgotten about Acorn 4. We've just pushed Acorn 4.5.8, which contains compatibly fixes for MacOS 10.12 Sierra. Acorn 4.5.8 is now compatible with 10.8 through 10.12.
This release is currently for direct customers only. For some reason the App Store has Acorn 4.5.7 in some sort of wedged state that we haven't quire figured out. We'll see if we can get that resolved sometime soon and get v4.5.8 out there as well.