Most of devices don't need externally visible IP address, so IPv4 is still adequate... The problem of IPv4 is that some larger blocks have been allocated to entities that don't need that many IP addresses, but because addresses are allocated in bits, each address block is twice as big as the next smaller one.

Even though IPv6 has been around for years already, there is still devices and applications that don't support it. Biggest problem for applications is that there is no standard C library function to decode IPv6 address represented as string containing both address and port. With IPv4, it is easy as there is only one ":" character and reverse search can be used to split the address from the port.