Phones with 3 and 4 SIM slots, support 3 and 4 SIM cards standby at the same time.
4 SIM standby simultaneously:
http://www.china-mobile-phone.com/2010-10/f160.htm
3 SIM standby simultaneously:
http://www.china-mobile-phone.com/2010-11/n8-g.htm http://www.china-mobile-phone.com/2010-11/f5-b.htm http://www.china-mobile-phone.com/2010-11/x97.htm