With just two of you and a 4:15 res, I think you'd be safe to start 5:45-6. 5:45-6 would allow you until 6:45-7 with the window to use it. If you would like to eat more leisurely, take your time, relax between courses, etc. then I'd push it back a little but my last experience with Ohana was everything was brought quickly and seems they want you in and out quickly to turn the tables.