US is in T4
UA is in T2
F9 is in T3

You will need to leave the terminal (and secured area) and catch the inter-terminal bus. The quickest was from T4 to T3/T2 is to head to the South side of baggage claim in T4. The bus will stop at T3 first, then T2.

From T2, get on the bus, and your second stop is T4.

I'll let others more knowledgeable tell you the best bus names to get on.