Elon Musk set about to have the largest positive impact on the world he could, so he started a space company and an electric car company. He believes we should colonize Mars so if some disaster befell Earth, some humans would survive. He also saw transportation as the area on Earth that needed to be revolutionized the most. He has the opinion that if it doesn't violate the laws of Physics, it's possible and he's going to figure out how to do it.
As for losing $4000 a car, they aren't literally losing $4000 every time they sell a car. Tesla is making a lot of money on each car. However, selling Model Ss is currently the only place where they are making money. All the R&D and building they are doing are all costing a lot of money. So if you take the profit from building cars, subtract all the expenses, and then divide by the number of cars sold, you're left with about $4000 a car loss.
