Currently domestic 747 service is usually done for repositioning after unscheduled maintenance or unexpected events — for example, when SFO lost a lot of capacity after the OZ214 disaster, UA ran multiple 747s LAX-SFO with about one day's advance notice.
A few years back there was a 747 doing a regular rotation SFO-ORD // ORD-SFO, but this service is no longer a 747. (You can still usually find an internationally configured 777 on this route.)
If you suspect that there's a 747 flight in April 2014, you can rapidly check schedules using ITA Matrix and the "/aircraft t:747" parameter:
This search returns no results (there is no UA 747 currently scheduled to operate ORD-SFO or vice versa on any day in April).