Always ask for a transfer/fare receipt, even if you are not going to transfer. Transfer/fare receipts are valid for 90 minutes.
sfmuni.com has a text description of the 4-line
4: Alternate service available
Weekday Service only. For service after 6 p.m. and on weekends, see 2 Clement and 3 Jackson schedules.
Buses and F-line: Always ask for a transfer, even if you are not going to transfer to another line.
