4pm dinner - seated no later than 4:30. 1.5 hours for dinner - brings you right up to 6pm. Travel to MK - can be up to an hour. 7pm arrival.
I'll be completely honest though - we did LTT in the MK at 4:30 before MNSSHP - we'd never do it again. It was too much for us. I'd do a big lunch close to my resort and grab snacks and eat CS at the party next time. That's just us though.
I would do a late lunch and grab a CS during the party if you need to. Try to get lunch around 2:30. They will let you into the park as early as 4 pm if you want to maximimze the value of your ticket.