As a Spark owner now, too, I can see the logic of Go 4 showing the default home screen on startup: it doesn't know what other DJI drones you might have and want to fly now, you have to select it yourself. (and remember, we're all having first world problems here, this is just one of many) At any rate, powering on the RC first (which is Mavic-only, so it tells the app, "Here's Johnny!"
) is a way to go.
I now usually power Mavic first so it starts grabbing sats, RC next, Go 4 last. Another benefit of doing this is that Go 4's status panel (the one you have to dismiss to get to Flight screen) will show current status, so if Mavic already locked sats, it will show GPS mode, unlike showing ATTI/OPTI if doing the other way.