And what's the non-urban area in MN-04?
Northeast portion of North Oaks. And that tiny detached bit of White Bear that's to North Oaks' northwest.
The western boundary of the Minneapolis - Saint Paul UA within MN-03 runs through Dayton where a ribbon along the Misssissippi is included, then along the Dayton-Champlin line, through northwestern Maple Grove, east-central Corcoran, southwestern Maple Grove again, northwestern Plymouth, along the Plymouth-Medina line (with two thin ribbon extensions into Medina, the longer northern one of which actually extends into Corcoran), through Orono along a very complicated line that leaves Long Lake and the parts of Orono southwest of it - and also a bit to the immediate north, around Dickeys Lake - within the UA but the territory to Long Lake's immediate south and southeast (reaching down to the shore of Lake Minnetonka in some places but not in others), as well as to the west, outside, and through east-central Minnetrista, with ribbon extensions to Maple Plain and Saint Bonifacius, both of which are wholly inside the UA.
At its western end, the district even includes slivers of two UC's - Delano-Rockford UC (territory on the western edges of Greenfield and Independence, bordering on Delano and Rockford themselves which are in CD6) and Saint Michael UC (The built-up part of Rogers. With a short corridor connection along I94 in Hassan - Saint Michael itself is in CD6, and considered mostly rural by territory. Most of the territory of Albertville is also in Saint Michael UC.)
Complex, isn't it? Its tracing the boundary of the built-up area (beyond a certain pop. density) in 2000. I'd blindly guess that Delano-Rockford and Saint Michael are really suburban rather than smalltown in character, and are considered separate Urban Clusters because they are too clearly divided from the UC by farmland.
Which means that this definition, applied to Frankfurt, would result in a pretty small UA...