Go to Setup mode (Cmd-1 or Ctrl-1).
In the Flows pane at the bottom locate and select (by clicking on) the flow that contains your imported MIDI. Now, with your flow selected, look to the Players pane, top left. Add your string quartet players by ticking the boxes next to each of them.
Now go to Write mode (Cmd-2 or Ctrl-2) and find your imported MIDI.
Select the first bar of the imported MIDI by clicking anywhere in the space between notes.
Bring your mouse pointer up to the very top of the screen until the menus appear (they’ll already be showing if you’re not in full screen mode).
Click Edit, then Select To End Of Flow. The whole of that stave should now be highlighted.
Click Edit again, then go into Filter, Notes in Chords, Top Note or Single Notes.
Then use Cmd-X or Ctrl-X to cut these notes.
Select the first bar of the Violin 1 stave.
Use Cmd-V or Ctrl-V to paste these notes.
Repeat Step 2 twice to fill the Violin 2 and Viola staves.
The Cello part will be quicker as you won’t need to filter (given you’ll only have the cello line left once you’ve filtered and cut the top three notes of each chord).
In future I’m sure Dorico will have comprehensive explode/reduce features, but I’m sure it’s taken me considerably longer to type this post than it’ll actually take you to complete the steps.