So as I don't confuse anyone....
I wasn't saying lat pull downs should be a substitute from pull-ups/chins. They should be an accompanying thing.
So, if you can only do 3 sets of 4 negatives, then that's all you do. That's what I would consider your "main" lift and the one you keep track of progress on. Once those are done, then you head on over to the lat machine so that you can actually work some volume.
There is nothing "smart" about a 3rd attempt deadlift.