Trouble with Ola Hallengren's @DatabasesInParallel parameter
After reading Ola's last comment I had a light bulb moment and figured what I was doing wrong and got it working.

Since I was just testing these commands out I never actually put them into an Agent job - I figured they would work the same way by just manually executing them at the same time. I just had all 4 of the job commands pasted in a Query window and would just execute that. I thought this would work but it appears that it doesn't even though they all showed the same RequestStartTime in the QueueDatabase table.

Once I created the jobs and set a single schedule I could see the RequestStartTime varies by milliseconds but the DatabaseStartTime is the same for all DBs.

The screenshot below shows the QueueDatabase table from my test today (pink) which was successful when I created an Agent job. The test on 11/8 (blue) was when I manually executed the commands in a Query window.



I asked Ola in an email if this functionality makes sense that it doesn't work when executing manually in a query window and below is his response:


  When you execute all the commands in one query window, you can never
  get parallel executions (as T-SQL does not have support for that).
  
  If you would like to test it in SSMS, then you need to us 4 different
  query windows, and then quickly start the execution of all of them.