I have been nursing my daughter to sleep. Before she could roll and once we worked out nursing laying down, we'd both lay in bed, she'd fall asleep and I would escape. Then when she started rolling I created a bit of a pillow fort but now she's definitely on the move so I will nurse her to sleep then once she's asleep, transfer her to the cot. With babywearing, have you tried a high back carry? You may still have issues, but I find that even with a lower back injury, I can back carry literally all day. And have done on many occasions. Front and hip carries I can only get away with for a very short time though. Another suggestion could be (assuming baby #4 won't go down in the cot for day sleeps) trying the pram?