I agree with Max
You need to wait for a stable version of Joomla4
then wait about 1 year for a stable version of VirtueMart and only then develop our own extensions to VirtueMart and remodel to J4
No, that is too long. When an RC for joomla 4 comes out,.. it may take 1-3 months until it is really released. Usually there is not only one RC. As far as I understood the changes for joomla 4, it is not a lot todo for us. Maybe I miss important points. But I do not expect more than a month for compatibility. I expect after some days an installable version which works on the first view. Usually extension developers are the right persons to push and to find and to solve bugs. The goal is to have a ready version for the joomla 4 release and that there was also enough time to adjust the 3rd party extensions.
https://developer.joomla.org/news/790-joomla-4-is-on-the-horizon-alpha-11.html"We would like to remind our users that Joomla 4.0 stable is not expected before the end of 2019. At present we are intending Joomla 4.0 Alpha 12 to be the last alpha of Joomla 4 and to present Joomla 4 Beta 1 in early October 2019."
As far as I know there is no beta yet. Joomla 4 comes maybe in summer 2020. But looks like the first Beta comes now. Of course people could try an installation of it and provide some bugreports or solutoins.
Here is the list of changes
https://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_4 Most of them do not affect us, or I rewrote it already the new way, because j3.9 provides already some j4 stuff.
The rest of your text is just sarcasm, which does not fit here. As I wrote above, when you want to start your development now, install the last vm core on joomla alpha latest. Currently I know, that we use one function, which is replaced by another. We can adjust core vm as long we do not need a switch.
When they release j2.5, they changed a lot in the final RC which removed a lot problems which took a lot time (at the end for nothing). So we wait until their whole frame is mainly ready and they are just on bugfixing, then is the best moment to start the real compatibility development.
And btw, some of the team already tried Vm on j4 in Juli.