Combine teff flour, barley flour, corn flour, self-rising flour and yeast in a large bowl. Slowly add water and whisk until no lumps remain. The consistency should be thinner than bread dough but thicker than crêpe batter. Cover and let stand at room temperature for 3 days.
Pour off the water that's risen to the top of the batter and reserve. Whisk the batter until smooth. If necessary, add the reserved water, 1 tablespoon at a time, until the batter is thin and pourable, like a slightly thicker crêpe batter. Cover and let stand at room temperature for 1 hour.
Heat a large cast-iron skillet over medium heat. Pour 1/2 cup batter into the pan, tilting and swirling to create a thin layer. The batter should spread easily. (If it's too thick, whisk in 1 tablespoon of the reserved water--do not add tap water.)
When small holes start to form in the batter, cover the pan and cook until the edges are dry and lifting up, about 45 seconds. Run a spatula underneath and transfer to a wire rack to cool. Repeat with the remaining batter. (If the injera starts to stick, brush the pan with 2 teaspoons canola oil.) Do not stack the injera until completely cooled. Serve immediately or wrap in plastic until ready to serve.
Tips
To make ahead: Injera will keep for up to 2 days at room temperature or refrigerate for up to 1 week.
