There's a rather impressive change log on the 4th and Mayor blog, but Jeff Wilcox calls out the big changes as :
- Quick Check-in now respects your Foursquare settings preferences for pinging Facebook & Twitter.
- New Live Tile Studio for organizing, customizing, renaming, and enjoying pinned places.
- Fixed bugs where places pinned to the start would disappear after a phone restart.
- Performance improvements and user experience tweaks.
- Integrates with Bing Local Search on the phone.
- Supports Windows Phone 7.5 features like fast app switching.
- Lists support from Foursquare, the new way for sharing information about places and tips, replaces “todos”.
- Maps of nearby friends.
- Maps from place pages will show your current location and the destination place
Fast app switching, Live Tile support, an app that continues to do what it set out to do, and then some more. This is what I love seeing in the app developer scence. And it already supports the "Expertise" badge modification launched in late October by Foursquare.
You can find 4th and Mayor in the Windows Marketplace.