Use the google search at the bottom of the page, you will find a ton of results. A 4 pound flat is smaller than anything I have ever cooked but here is my advice.
Rub it and cook on the UDS at 275 or so fat side down. Wrap in foil at about 160 degrees internal. Start checking for tenderness at around 195. When a probe slides in without resistance wrap it up and put it in a dry colloer for an hour or two. Take it out and let it rest aor about a half hour then slice against the grain. You should be good to go.
Even just salt and black pepper makes a great rub. Good luck.
BWS Party, UDS, Yellow Thermapen, Gasser