If it strikes your fancy 4 hours is definitely enough to hit up the Millbrae In-N-Out Burger at SFO. Just take the BART one stop to Millbrae and you can see the sign for In-N-Out from the station after you climb up the stairs. Get your order to go if you're worried about time and head back to SFO and your connecting UA flight. The nice thing about the BART machines at SFO is that they'll allow you to charge small amounts to your credit card vs. the minimum purchase amount at the other stations so put just enough for the return ticket on the card ($3).