On iOS I paid for version 3 the full price. Not a trivial amount. Now with 4 it is asking me to pay again for editing functionality and version 3 is wiped from the App Store. Either allow me to keep my app (because I am not interested in paying again, Im not a power user) or upgrade me to 4 for free.
Is this for ipad or macOS? Audulus 3 is still plenty usable, and it’s up to you if you want to keep it. And IIUC, you should be able to run Audulus 3 and 4 in parallel.
Audulus 4 is the product of 5+ years work – sorry, not a free update. If you want to get on board and enjoy the fruits of these labors, great! I don’t think you will be disappointed. All your Audulus 3 patches should work (though GUI stuff might get a bit jumbled because of the rewrite from the ground up of the graphics engine).