A four to five hour Greyhound bus trip from Los Angeles to Tijuana will cost you $26. Autotransportes Baja California, ABC, travels from Tijuana to Cabo San Lucas. However, the travel time is roughly 20 hours, and the cost of the bus ticket is often comparable to flight costs. If you can only be away from Los Angeles for 4 days, the travel time might impede on the chance to experience Cabo San Lucas.

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I hope this has been of some use.