Look at these maps of how each precinct voted in the Salt Lake County Mayoral race and the CD 4 contest. In the County Mayor’s race, the darker the blue, the more Democratic the vote, while the darker red precincts went more Republican.
Here's the Salt Lake County Mayoral vote:

As you can see, there’s a pretty distinct line right across the Southwest part of the county.
That line persists in this precinct map of the CD4 race between Jim Matheson and Mia Love.

One note about the CD4 map - it reflects the presence of a third party candidate. The very light red precincts are those Jim Matheson lost by less than 2% of the vote.
Justin Miller, Ben McAdams’ campaign manager, provided the maps to Utah Policy.
A few thoughts:
The really Democratic areas are “landlocked” with no real room for expansion. There’s not a lot of space for new voters to move in. Conversely, the Southwest corner of the county is booming with development. Young families, who tend to skew Republican, have plenty of opportunity to find places to live there.
Republicans are going to have to find ways to do much better in Midvale, Murray, Cottonwood Heights and the northern and western parts of the county. You can see some red precincts here and there, but they are basically surrounded by deep blue.
Democrats are facing a huge problem in Herriman, Riverton and South Jordan. Those areas are solidly Republican and are growing quickly.
One more thought on these maps. It looks like Democrats were successful at rallying the base to the polls on election day. Had Republicans been more successful in riding the “Romney Tsunami,” those precincts should have been a much lighter shade of blue.
Parting Thought:
“It’s like a drug, the feeling you could make a difference...” ~ Richard Ben Cramer


Latinos, transplants, and socially liberal Utahns will fill in the areas as conservatives move south and west. As I said, the voter balance may not shift much but Republican legislators and maybe a couple of county councilmen should worry about it.
The analysis is probably more true for CD4. Because SLCo is such a small slice of the overall area, it will be tough for gains in West Jordan to make up for growth in all of western UTCo.
As the northern area increasingly becomes Hispanic, the population density will increase. Many Hispanics tend to live in extended multi-generational families living under the same roof. Some families span four generations. When you include the higher birth rate among Hispanics, the Democratic population is likely to grow substantially over the next decade.
In addition, areas adjacent to Hispanic neighborhoods will likely change from white to brown. Hispanics will seek to live in communities which provide the services that they seek. Regretfully, a growing population of Hispanics will trigger "white flight" which will further diminish the number of Republican voters.
Over the lifetime of the Fourth Congressional District (10 years), it will become easier for a Democrat to hold the district.