IMHO 4AM is too late to start if you want to be serving at 6PM. Using Foamheart's 2 to 2.5 hour per pound guideline(which I believe to be fairly accurate if you cook at 225°), then you should start at 1 AM at the latest. A start time of midnight or 11PM may be even better, given that you may encounter any number of difficulties, such as wind, low ambient air temps, or even the ubiquitous "faulty temp probe" that can make cooking at low temps problematic when you have a hard deadline.
Unfortunately logical1's advice is incorrect, 165° is the temp at which the butt should be foiled and placed back on the pit to finish cooking, not the temp at which it is to be rested in a cooler. Since you do not plan to foil simply cook until the IT heads north of 200°, as Foamheart, suggests.
One thing that you do not want to forget is to have a spare tank full of propane as a back up.