No, 3G radios will not work with 4G signals, just like a Verizon phone will never work on AT&T’s network. Some phones coming onto market very soon, however, will likely have a 3G AND a 4G chipset, and so will work with both networks.
Expect those features to be heavily advertised
Obviously Yes, in near future 3G mobile devices will work with 4G Networks. Number of research programs and interworking analysis are on going through LTE (Long Term Evolution). There will be a mediator between 2G/3G and IP Devices to convert the message formats. All message flow will be through IP and service provides are keen in providing number of services but few penny needs to be provided from our packets.