There are actually several very nice wineries in the Paso Robles area if that is what you are looking for. You might check at your hotel and find something that will not require driving over to SLO.
On the other hand, SLO can be a nice place to visit as well. You said "the 4th of July weekend," but if you will arrive on the 3rd, a visit to the SLO street faire/farmer's market that evening might be nice. Thursdays only.