The DB4 is a UHF-only antenna (channels 14-69). It's not really designed to pick up VHF channels (2-13).
To pick up KGO (real channel 7), you can either add a high-VHF (ch 7-13) antenna to your current setup, or replace your DB4 with a "combo" antenna that can do both VHF and UHF. You don't have any channels in low-VHF (ch 2-6), so a high-VHF antenna is all that you would need. The Winegard YA-6713 and Antennacraft Y5713 are good antennas to consider.
If you want to add a VHF antenna to your existing DB4, you should combine them through a UVSJ diplexor (like
this) as opposed to using a standard splitter/combiner. The UVSJ will merge the signal with minimal loss and mutual interference between the two antennas.