If you are within 60 days of check in (or whatever the time period when online check in starts), you can find it easily:
Link your reservation on
MDE (using the 6 digit/letter number Orbitz gives you immediately upon booking). Click on the "online check-in" link near that reservation in your MDE. (I'm using a laptop - not the iPad app.) Once it loads (the page that has your arrival time, room requests, etc.), look up in your URL bar thingee, copy the entire, long, URL of that page. Then paste it into a Word (or whatever) doc on your computer. It has the '4' number!
Of course, most people probably aren't booking within 60 days like me, and thus want it sooner. But I thought that might help someone.
Also, the last 3 reservations I've made, Disney sent me an email letting me know it was time to choose FP+ - for Orbitz reservations. That email had the '4' reservation number! (didn't in January when I booked a room via Orbitz; did in Feb. and March.) Go figure. Again, this would be 60 days before check in.