@Blacksheep: He means that, if you reprogrammed the Core to something other than the Tinker software, trying to use the iPhone app to control the pins will result in 404 errors. There are two different things you can do to get Tinker back onto your Core so that you can test its connectivity again.
First, if your Core is currently connected to the Spark network (breathing Cyan), you can go into the Spark Core app, click the list icon at the top left-hand corner of the app, click the arrow drop down next to your Core, and press the “Reflash Tinker” button. Make sure that the Connected: status reads “Yes”.
If that is not working, you can complete a factory reset for your Core. It’s really simple, and reverts the Core back to the firmware it was programmed with when you opened the box.
Try a factory reset. Hold down both buttons, then release the RST button, while holding down the MODE button. The LED should begin flashing yellow. Continue holding down the MODE button until you see the Core change from flashing yellow to flashing white. Then release the button. The Core should begin flashing blue after the factory reset is complete.
Let me know if any of that works for you!