in theory, e46 coilovers should fit but just to be safe i would look for coilovers marketed for a z4 in case the z4 specific version has different dampening characteristics. easiest way would be to go to www.realoem.com, look up your car's suspension part and then do a cross-reference to see what other models it shows up for. if it also pops up for an e46 then you know it's interchangeable.