This isn't much of a surprise: the Alfa Romeo 4C was engineered from the outset to lose its roof eventually, so the 2015 4C Spider is really just the fulfillment of a long-standing plan. That's not to detract from it in the least: this is the 4C that might make the most sense.
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Here's why: it's not much heavier (22 lbs is the open-roof tax), rear visibility is much better with the fastback lid removed in favor of a (dare I say it) Elise-like buttress-and-slit-window arrangement, and it looks great. Everything else is basically unchanged: the carbon fiber monocoque, the 1750-cc turbo engine making 237 hp, and nearly identical 0-60 time and top speed.
It will come at a premium, though. Alfa wouldn't speak to what they'll charge for the 4C Spider specifically, but expect the Spider to carry a significant premium over the fixed-roof variant, partly because Alfa expects most of its Spiders will be high-spec models with plenty of options.
Here's one problem: Although Alfa is adding this second 4C model to its lineup, the company definitely isn't expanding its production capacity. Due to the car's labor-intensive production–somebody's got to hand-lay all that drool-inducing carbon fiber–the Maserati plant that cranks out the 4C can only produce about 3000 units annually.
Viewed one way, that means every Spider that Alfa produces means one less fixed-roof 4C for the world's hungry buyers. And with most dealers waiting a full 18 months for their allotted Alfas, the Spider is poised to boost demand (and transaction prices) without helping supply.
But if you're in the market for a carbon-fiber, mid-engined sports car with infinite headroom, and you don't want to spend six figures on it, it's not like you've got many other choices.