I wouldn't do a 5:15 at Tusker House. Since you can get into the MK with just a party ticket at 4:00 it would be better to be in or near the MK by that time. I would choose an early dinner at a CS at one of the nearby resorts or CS or TS inside the MK.
If you are planning to line up for one of the popular meet and greets (e.g. 7 dwarfs, Jack Skellington or the princesses with their princes) the lines start forming around 5:30-6:00.