Model S equivalent? I'm not aware of any car that exists that is a Model S equivalent from any company.
Or are you talking about a press release? Press releases have zero motors....
What I'm saying is that we shouldn't talk about other cars as if they exist when they don't. I'm tired of concept cars and you all should be too. Show us the goddamn cars already, Porsche.
Anyway, to your actual question: the reason Tesla started with one is because it's easier. Less parts, less complexity. There's also the issue of unsprung weight, which traditionally is thought of as a bad thing (but I've seen some newer information which suggests it doesn't really make as big a difference as tradition suggests). The thing is, the Tesla is a real car, not a concept, not a press release, not a one-off, an engineering exercise, a showpiece, etc. An actual car that people have to build and buy and own and drive and service. So far, almost every electric concept has a bunch of motors on it, but not a lot of final production cars do.
Will Porsche keep theirs? Well, *if* they even make the car, *maybe* they'll stick with 4 motors, as complexity and difficulty/cost of repair has never stopped Porsche from doing anything before, they do have more engineering prowess than most of the other car companies out there (thinnest kid at fat camp), and 4 motors *could* provide performance benefit - you could eliminate the half-shafts and simplify the gearbox by removing the differential and therefore possibly save weight (and therefore space, which means better control over center of gravity), and also you can do "torque vectoring" and run the inside/outside wheels at different speeds, which helps cornering, and would be much better even than an expensive, complex and comperatively dumb limited-slip differential (which the Model S does not have, because those are primarily used in racing applications).
So, if cost and complexity are no object, if scaling drivetrain production is not difficult either because they're selling less units or they have more resources available to scale up production more quickly, then maybe 4 motors would be nice, because it would provide performance benefits. But it's really not necessary, and that's why nobody does it (that and nobody else is making performance EVs...because they're scared of making their high-margin performance ICE cars feel like junk).