…I had one card in my hand, with Play 4 in effect. I played my card, an Action card which was then discarded. I wanted to claim to able to get 3 new cards because “Get On With It” which was on the table says I could since I had discarded my hand and had 4 – 1 = 3 plays left.
A: In order to take the option to Get On With It, you must be sacrificing (at least) one of your Plays, and you must be discarding a hand of at least one card.
The most obvious issue is that, at the point when you wanted to Get On With It, you didn’t discard your hand. You played an Action, and now your hand is empty. You have to have something to discard in order to discard something. Your hand has to exist in order to be discarded.
The second issue is almost a side effect. We would not consider you to “have plays left” if you have no cards to play. In this case your first play WAS your final play, so you can’t take this option because it’s not before your final play. In order to have a final play, you have to have a card to play.
The whole thing follows logically, since the card/s you could have played – but didn’t – will be remaining in your hand, and therefore among the cards you’re throwing away.
See also: Is Swap Plays For Draws limited by the number of cards you have in your hand?