It was an option to have them but no idea about how they are triggered, would imagine they automatically swing in upon locking the car. Its shown as a £160 option on the current bmw car configurator option code 313.
I have girlfriend-folding mirrors. Every time I want them folded I call her......"hun... did you forget something????? .... thaaaaaank youuuuu." Works great...and you don't have to push any buttons.
the German http://www.racing24.de website sells 'Spiegel Anklapp Modules' which are said to be Z4 compatible :?
for cars that have folding mirrors available, the module triggers the mirrors to auto fold when the ignition is turned off...
http://www.racing24.de/shop/product_info.php?info=p20206_Spiegel-Anklapp-Modul--IP-SAM-1-0.html&XTCsid=176e7498915f5ad788e0a498548cc008