Composition is notes and placement, music is duration and volume. There never should have been one without the other. Anyway, 4 is pretty cool in the note editing and it is in Write mode. But I can’t switch until my current job is complete.
Loading Dorico 4 does not wipe out Dorico 3; so you should be able to host both on your machine as long as you keep the existing eLicenser (dongle or internal) connected/active.