In my single-player, superflat world, I went a few blocks away from spawn, but when I made the 5x4 standard portal and set fire, I couldn't go to the nether, although I had in previous worlds.
You need to be on infinite, ok
Oh, looks like your nether portal is not big enough.
A nether portal is built as a frame of obsidian (4×5 minimum, 23×23 maximum). http://minecraft.gamepedia.com/Nether_portal
You need 4 blocks width, 5 blocks tall of obsidian. The four edges are actually not required. And you need to light it up with fire (flint and steel)
If you get it right, you should have a nether portal with purple effect and some sound effect.