4 campaigns at once is really too many I think; I'm planning to rest the weekly online Ghinarian Hills one (5e S&S) for a bit until I can conclude the fortnightly Loudwater (4e Epic) one in August. I can't rest my weekly Mentzer Classic Karameikos game since I love the players too much & my son plays in it & would kill me. Also one of the players is a brilliant instigator, her renegade Claudia Morrigan Thief PC is returning with plans to wreak horrible revenge on erstwhile Cleric PC comrade Roseanna 'the White Dame' and I'm keen to see how that plays out. My 5e Shattered Star game is trucking along nicely fortnightly, wisest thing there is not to mess with it.
To the extent I have a secret for running 3-4 games, it's minimal prep - only the Pathfinder AP ones ever take much work. Taking good post-session notes is the main thing I do. Generally it's just as easy to create material at the table as beforehand, and simple published adventures (eg Basic Fantasy stuff) make just as good a canvas as complicated ones.
A few good resources are important - for Wilderlands that's the Ghinarian Hills campaign page and the NPC stats blog. For Karameikos it's GAZ 1 Grand Duchy of Karameikos. Plus published old school modules for both, simpler the better. The d6 is my friend, rolling to see if something happens - on a '6' X happens, which might be a wandering monster table roll but equally well could be the Griffon Riders of Highaven swooping in to aid the heroes' battle, or interception by the undead war fleet of Neo-Nerath - whatever looks plausible.
There is a lot of not sweating the small stuff, plus a lot of "God I wish I'd taken 30 minutes to do some prep before running this session".
In general running SS in 5e has been far far easier than running Crimson Throne with Pathfinder. Paizo's tendency to include obscure monsters from Bestiary #5 without bothering with a PF statblock is annoying, considering how much space they waste on other stuff. Eg the Fiendish Seugathi in book 2 has no parrallel in any D&D creature. But this is still a minor issue compared to the brain-scrambling effect of trying to run a 3-page 3e/PF stat block off the page in Pathfinder rules.
I think in future I may just substitute entire monsters, eg that CR 8 Seugathi could have been any old demon or devil of similar CR. The trick is to select a monster of similar-ish CR, rather than one that looks the same but is vastly weaker or tougher.