I tried v4.0.0 and ran into printing problems on my mac. (See also https://www.mydraw.com/forum/71/Version-300-does-not-print-in-landscape).

I found the download link for the previous version (https://www.mydraw.com/downloads-do-download?downloadid=70126), got the setup pkg and launched it. All seemed normal. Still, v4.0.0 launches.
Then I uninstalled the program (using the usual manner in mac: open the Applications folder in Finder, move the app to the Recycle bin), and installed from the above-mentioned download. Still, v4.0.0 starts up.

How can we downgrade?

I uninstalled V4 using App cleaner - which does more than just move the papp to the garbage. As a result its was completely removed and I was able to download and install the latest V3.

BUT I can only get it to run in demo mode. The web site refuses to allow me to register it.
 
P.S. For me V 4 just blows sky hinge every time I touch it.  Sometimes if I start a new diagram, it works for a while and then just goes boom with no error message. If I try to open an old file it just blows up during the open.