John at The Ancient Gaming Noob gave a detailed account of the destruction that occurred across New Eden by the participants of Hulkageddon 4. Overall, the numbers are down from Hulkageddon 3. Why? Here are my top reasons why?
Full API handover required to participate - This is possibly the biggest reason. For Hulkageddon 4, Helicity ran an API verified killboard which required participants to give him their full api key. Not only did the cut down on the fraudulent kill mails that were submitted, but reduced a lot of manual work and resulted in more accurate counts. Also, many people are wary about handing over something as sensitive as their full api key to anyone they cannot punch in the face. Less participants = less kill mails.
Incursions - The latest big thing in EVE is fighting incursions. Not only are carebears fighting Sansha nation, but those who might gank miners are having fun fighting the local defense forces and ninja looting the battles instead. Combined with changes in insurance payouts, Sansha incursions give a bigger chance of big isk payouts than ganking miners and possibly winning a prize. The fact that the incursions have the beneficial side effect of taking out mining bots, thus reducing the killing fields for Hulkageddon participants, also helps drive down the numbers.
Carebears changing the way they play - If I recall correctly from when the first Hulkageddon was launched, changing the way that carebears play is one of the goals of Hulkageddon. That definitely occurred this time as the anti-Hulkageddon forces became more organized. But in doing so, they fall for Helicity's sinister plan to turn them into PvPers.
RIFT - Related to changing the way carebears play is just not playing at all. And Helicity just had the bad luck in December of setting the dates for Hulkageddon 4 during RIFT's open beta weekend and the head start weekend. I thought that the concurrency numbers were down a bit when I would log into Eve, but I can't really tell by looking on Eve-Offline. I know that I spent a lot of time in RIFT instead of playing Eve and I'm guessing a lot of others did as well.
Time of year - This is a reason that I've read Helicity give on the forums, so I will list it here. However, I am not buying into this theory, as I heard SOE & SOE fanbois give this reason for the slow decline in the population of EQ2. Just based on my personal experience, more players have been logging into EVE this winter than during the summer of 2010. I think that number went down during Hulkageddon, but the reason wasn't the time of year.