I got 4-axis machining debugged in an emulator with my DIY CAM program!
It does parallel and crosswise -strategies using any 2 linear or combinations of a linear and a rotational axis and can optionally use the rotational axis to have the cutter follow the surface normal for a perfectly smooth finish (not just 3+1 axis but real 4 axis milling)!
There’s still a lot of cleanup to be done. e.g. the test if the tool collides with the part is done only once but not a second time after the inverse kinematic of this 4-axis strategy. There’s also a lot of hard coded stuff that belongs into proper settings-dialogs,…
but it works!