Grognardling has done a great job outlining some rules for guilds in a recent post and it's inspired me to find some way of incorporating guilds into 4th edition using its inherent modular mechanics.
Guilds are likely something that will come into play in PCs' downtime, but there's no reason they can't play a part during the adventure. I think treating guilds like items is a good way to go in 4th edition, as they can be as simple or complex as you like. The stat block gives the bare bones of the idea, which can be fleshed out through roleplay.
Con artists, pick-pockets and burglars walk the halls of the Thieves Guild, swapping tips and receiving training from the Guild Master.
Entry Cost: 5,000gp
Prerequisites: You must be a Rogue of at least level 5.
Advantage: You gain a +2 to Thievery checks.
Power (Encounter): Gain a +2 to Stealth checks until the end of your next turn.
Power (Daily): Free action. Gain a +20 to Thievery checks until the end of your next turn.
Traders from around the kingdom join together in the exclusive Merchants' Guild to teach each other about this noble profession.
Entry Cost: 7,000gp
Prerequisites: You must be at least level 5.
Advantage: You gain a +2 to Diplomacy checks.
Power (Daily): Free action. You automatically get 50% off the value of one bought item. Alternatively, you persuade the buyer to purchase one item for 25% more than the market price.
Veteran ocean travellers come together to trade stories and gain access to vessels.
Entry Cost: 10,000gp
Prerequisites: You must be at least level 11.
Advantage: You gain +2 to any check related to sailing.
Power (Daily): Free action. When at a port, you gain access to a small 6 person vessel for free for 2d6 days.