I agree with dobfarm. For it to be a 6, it would be necessary to explain what the bottom corner (shown by the red line) is.
There's an apparent 4 above with a near-identical extension of the corner. Yes, the line in question has some soak-through, but this corner is clearly not soak-through.
If you can find a 6 on the page with the same corner, then go with 6. Otherwise it's clearly 4.