It's been four years and one month since the last episode of thirtysomething. But who's counting? Creator and exec producer Ed Zwick has said ''there is still the notion that we will do a reunion,'' but the old cast is busier than ever. A number of cast members are now card-carrying members of the Directors Guild Ken Olin (White Fang 2), Peter Horton (The Cure), and Melanie Mayron (the upcoming Baby-Sitters Club) and Patricia Wettig and Timothy Busfield will each appear in new TV series this fall. The good news is that the cast is up for a trip down memory lane. ''We all check in like we were war friends,'' says Mayron. ''It would be great.'' So what's the hitch? Zwick has said the timing is just not right, but Mayron sees another dilemma. ''The biggest battle,'' she says, ''is who would get the directing job.''