Hope someone can help me out, and thanks in advance.

I will be travelling to Switzerland via London in March next year. The itinerary planned is:
SFO to LHR (arrive 11:25) by United Airlines, then connect to:
LHR to ZRH (depart 15:35) by British Airways.

Will 4 hours be enough time? The 2 flights will be separate ticket purchases. Does that mean I will have to go through arrivals, retrieve my checked bags, transfer to another terminal, check in my bags again, and go through security?

The reason I will have to connect is my overall trip will be to Switzerland first, then to the UK, then fly home to San Francisco (If I booked a one-way ticket from SFO-ZRH, then another one-way LHR-SFO back, the prices are astronomical). It's just a ton cheaper to do a roundtrip ticket SFO-LHR, and separate ticket LHR-ZRH.