OK, so this I wasn't expecting.
I got a surprise PM from Steve (Rukario) and it looks like someone gave me a gift card. I immediately accepted it and got upgraded to a Tier 5 Supporter.
So yeah, I keep all the perks I had as a Tier 4 but I now get a shiny new email of my own and I even got a new emblem to add to my collection.
I can't believe I've only been Tier 4 for close to 4 weeks and now something mysteriously comes out of the blue and takes me up...