P0204 injector failure code, then went away?

So I got a P0204 and P0304 codes for injector failure and cylinder misfire. I went to move injector #4 to another cylinder just to test to make sure it was a bad injector. Funny thing was when I started up the engine after moving the injector I don't get get anymore codes? Can something I did in removing the rail, cleaning the intake manifold, and moving the injector make the problem go away? Or should I carry an injector with me on long trips just in case it is about to throw a code again for real?
thanks