Between 4 and 4Z definitely a tough call, and I'm fully understand if one person prefer one to each other.
One definite plus point for 4 is it has more euphonic sound that super easy to enjoy. While at 4Z, the more accurate midrange tonality can be an advantage too.
Let's say I listen to 10 female singer (Norah Jones, Diana Krall, Patricia Barber, Jennifer Warnes, Inge Marie, Katie Melua, Rebecca Pidgeon ,Sara K, etc, well you know, the usual "female voice club"), while both headphones perform really well, but I can listen truer to the singer with 4Z.
With 4 I can differentiate each singer easily, but there's always that LCD-4 kind of character that stick with each singer's voice. Some may called this as 4Z has less coloration than 4 in midrange section. But, at the same time, like I wrote above, 4Z also has less soulful effect compare to 4.
note: both are LCD 4 series, so actually the differences are not that much.