I think Ask The Audience is the best lifeline. It's so often wasted.
For a really difficult question, if you Ask The Audience and three answers get less than 25%, it's highly likely that the fourth answer is correct - even if it only got something like 28%.
Things get very dodgy when two answers have more than 25%, e.g. 33%, 40%, 17%, 10% ... bad; 28%, 24%, 24%, 24%... good.