In the ever-competitive minivan market, the little differences can make a company's year, or make it miserable. The bigger differences, like the number of doors, can be even more significant. Which is why the Ford Windstar, late to the four-door party, has brought along a surprise guest this year: the Family Door. It's hard to have any discussion about the 1998 Ford Windstar without someone mentioning That Big Door. Ford's official name is the Family Entry System. Ford insiders call it the More Door. The Windstar still has only three doors--a major disadvantage since Chrysler and General Motors front-wheel-drive minivans all offer an optional sliding door on the driver's side. When Ford was designing the Windstar, its market research showed that minivan buyers didn't really care about having a fourth door. Wrong. Parents find the fourth door makes getting kids, especially little kids, in and out of the van a lot easier. It is being ordered on something like three out of four Chrysler vans. The Windstar will offer a fourth door when it's redesigned for 1999, but the big door is the quick fix for 1998. By making the driver's door six inches longer and offering a driver's seat that slides and tips forward, you can now get kids in and out of the back on the left side of the van. Obviously, one door is simply not the same as two. But that doesn't change our overall impression that the Windstar is a fine minivan. It remains convenient to use and pleasurable to drive. A star performer in government crash tests, the Windstar also offers plenty of room for big families and lots of stuff.