Note: This was intended as a comment and then got too long. But first of all: thank you for these words Dominic! :)
I would like to see 4 mods for GD, and as you mention, maybe a mixture of oldies and new blood (rotation is good, so of course if all new mods are elected we ancient decrepit ones will happily share our knowledge on the fun world of internet trolls, spammers and flame-recruits).
But I think the main reason for having 4 mods instead of 3 would beis the amazing increase in activity the site has had in the last month. Just taking a quick look at the stats, we have easilytripled doubled(true story) the number of visits, questions and answers. And I am seeing at least double the number of flags I saw before, for sure.
I think 3 people can probably handle the current level of activity, but I see GD.SE growing even more in the next months, and thatso the site would definitely requirebenefit from an extra person. Moderating is not just about the flags, it's alsoor the commenting, the editing and editing. It's also everything above the janitorial stuff (meta, projectslike posts on meta, etc)events, even keeping the chat 'alive'. It all just takes time, but the less time spent on handling flags and such, the more.. time there will be to discuss and plan activities. Also, the more the merrier :)