Modification to 4k Milestone (Patreon)

We may never reach that point, but just in case we do, I've added "a second pinup image of Dan's choosing" to the 4k milestone.

Because, frankly, I want an excuse to have more than one of those per month, but I still have a backlog to work on, so I figured I'd best attach that to the NEXT-next milestone for now ;)