I'm going to implement network features for the game I'm currently working on, now I'm wondering if I need to implement IPv6 support or just having support for IPv4 is enough?
Your question, as specified, is far too broad to be answered. The answer depends on knowing what you're actually doing with networking.
For example, if all of your networking is done over HTTP and similar protocols, then IPv6 support is sufficiently simple that it's not really a problem. You could probably slap it together in half a day or so. In this case, there's really no reason not to.
However, if you're doing peer-to-peer networking over UDP, where you're trying to squeeze every bit into each packet, then IPv6 is not a minor thing. It has a larger packet header than IPv4, out of necessity. So that means less room per-packet for your data. That can mean that you need to employ greater compression or re-evaluate how much data you send or whatever.
I will say this: you should leave the option available. Look at the design of your networking system and make sure that you're not doing something that will, from an architecture perspective, make IPv6 implementation harder than it needs to be.
IP addresses should be within a type that could store an IPv6 address, for example. Your interface to the socket layer should be abstract enough that the basic act of sending packets (if your app does that) will be the same over IPv6 as over IPv4.
IPv6 is coming, and if you're going to actually release a game with networking, at least having a plan to support it is a good idea.
Short answer for this particlual case would be "No". Long answer would be "Is it likely to determine your game's success or failure"? At the time then IPv6 support will be a must you can update it quite easily, isn't it?
If it's just a matter of personal interest you can absolutely try to implement IPv6 support. But, for me, it's better to spend that time on actually making game better.
P.S. I'm almost sure that most big major game manufacturers already include IPv6 support for their games, but that's another story.