Originally Posted by Lux Flyer

Based on the second part of WineCountry's post, due to the continuous pricing implementation - K4 in iOS "expert mode" display doesn't necessarily mean there is true 4 seats of K availability and you need to look at the flight in a GDS (or provide the specific flight/date you're looking at and others can look for you) to see the true inventory allocations. Due to continuous pricing UA's system might be willing to overbook the K class and sell a continuous priced fare for 4 people at a price differential above the actual K fare filling (which is why it is showing K4), but if you're only getting the actual fare filing/price when you search for a max of 2 people it is likely the actual booking inventory is K2. Again, there are plenty of people who could check this, but would need to know the specific flight/date in question, otherwise we're just guessing at the actual cause based on the limited information we have.