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Further: add treasure.
Not just monetary-value treasure. Except to collectors. So, for example, they can donate stuff to their organization in order to get prestige awards (like having to donate 10,000 gold-worth of stuff to get anywhere with the Pirates! Agh!), so it's stuff you've described (ancient spear heads, lost tablets, other non-magical, but masterwork and preserved relics like pottery) that the PCs can't really use or dispose of, so the best thing to do is just hand it over to their society for study.
Also, drop in magic items. Especially nearly-pointless (adventure-wise) magic items, like self-cleaning plates and utensils (constant prestidigitation), clothes that are still good after 10,000+ years (constant mending), and things like working (or not) magic-toilets or magical tooth-brushes or whatsoever you can think of to make the place "nifty". Throw in magical baubles ("It's a floating silver ball! It follows you around after you touch it, but not after you touch it again ... that's all it does.") or magical effects (you sing "I'm a little teapot" and it shows up on the wall") that serve little power-purpose, but really make it a fantastic city to explore, see, experiment, and check out.
Also, drop loot. Push. TREASURE. Give the PCs some breaks here and there. Let them find nifty things they can keep and/or use. Mastework stuff is great here. Use their group to "liquidate" their unwanted stuff via long, over-land caravans.
FINALLY (I promise!):
Play the fourth adventure in the midst of the third. The best time to do this is, if/when a battle takes place in the boggard area (farming district), have some people disappear. Have some not come back. Send the PCs to investigate. Instead of a pirate, they find a pathfinder down a sink-hole. Play that part of adventure straight. Alternatively, have the Rakshasa (with his shiny new cape of the montebank you just added) dimension door NORTH, and run into "a random island", attempting to hide in a cave... and voila, the next vault is found. Also, potential new ally for the evil guy.
These are a few ideas of how I introduced the sub-plot of adventure four before adventure four.
Be ready for them to explore/defeat/loot the place quickly.
That's it for now!