Err I think you answered your own question? If you're asking for the time zone of GMT -4, it would be GMT -4. As per aquariums, I think it's been screwed up for a while. I think the best solution would be to find the time disparity between the actual time and gaia's time and use that to predict when your tank will be glowing next.
i used to have it on 5GMT before daylight saving times but time changed to the same time as usa
but ever since then its glowing at hours that i cant be on to play and when it sayd it will glow at this time it just dosent glow so im just curious and concern if its me that have the wrong GMT