I have BC Coilovers on my Z3 and after corner weighting and camber adjustments very very happy. The adjustment available has allowed me to get itbjustbyhe way Inwant interns of looks, enough clearance and handling. The adjustable damping settings have been used plenty in the beginning to find the damping speed I wanted on my usual roads.
It was crucial to get the right spring rates - initially started with 6kg all around which was too soft at the rear (spring bottomed out). Now changed the rears to 8kg and it's now spot on.
I haven't upgraded the ARB's yet. Would like to but I think for road use its fine as is as I don't feel like it rolls at all. Perhaps on track I'd notice some roll.
@Brian H runs BC's and updated ARB's on track so can comment I'm sure.
What is the z4 setup like at the rear - is it swing arm with seperate shock and spring or is it a true coilovers set up?