Since most of the active members have only been here for 2-3 years and still have a few to unlock it probably doesn't get much consideration but I bet as we all get closer to 5 this conversation will pick up.
You lack a vital quality found in all Sith. Sith have no fear, and I sense much fear in you."
I'm sure Eric will add a 5, 6, 7, 8 ect badge. He probably hasn't gotten around to it yet. Currently there are 33 members who have the 4 year badge. Out of those 33, only two of them are active on the site. I think Eric is waiting for some more active members to get to the 4 year before he makes the 5 year badge and on.
I find it interesting that Eric made the badges when the site was already 6 years old (mind you, it appears to have gone through quite a few major changes), but only made badges for years 1 - 4. Either way, it has been two years since then, and 33 people have the 4 year badge, so, some level of symmetry would suggest at least doing a 5 year, and then every 5 years from that point.