In the hierarchy of collector cars, convertibles are kings. Drop down a rank and coupes attract the most admirers. At the very bottom of the list, rarely allowed into the palace garage, are those ubiquitous, hard-working, uninspired drays known as 4-door sedans. You can buy one in perfect condition for a fraction of what you'd pay for a nicely restored convertible. Yet the time is coming when a 4-door sedan will be coveted by collectors and big bucks are exchanged for the privilege of owning one. Here's why: The modern 4-door sedan is on the way out, being replaced by so-called crossovers which typify the North American family car of the near-future. In Europe the tall, 5-door hatch with acres of interior space contained in a tightly-drawn and smart-looking exterior has already found favor with most buyers. In the UK - among the world's most competitive and diversified markets - Nissan has declared its intention to stop producing and selling 4-door sedans. Combine this trend with the endlessly soaring prices of classic convertibles and coupes and you have a scenario wherein the 4-door becomes admired and desireable. Buy now and remember, you saw it first here.