Look the rest of the truck over real close. If there is rust in one area like cab corners pay attention to behind the box below the tail lights, wheel wells, front core support, and cab mounts. If serious rust is there walk away in my opinion. You shouldn't have to travel far to find another f150. As with any used truck plan on a few thousand bucks in repairs and if you find it doesn't need it all the better. For reference in Idaho a truck like that would go for 2000-3000 unless it was spotless.
you can't even hardly get a decent used golf cart or 4 wheeler for $4k, all about perspective with used anythings. I'd be leery goin cross country to save a lil bit when the gas alone will eat ya up in one of these trucks. Good luck with it, your gonna get teaser ads left and right posted on this with "better deals" from all over lol