What is the most likely meaning of the phrase "uncomfortably on all fours yet unheeding his discomfort" in the passage below?
Jack was bent double. He was down like a sprinter, his nose only a few inches from the humid earth. The tree trunks and the creepers that festooned them lost themselves in a green dusk thirty feet above him, and all about was the undergrowth. There was only the faintest indication of a trail here; a cracked twig and what might be the impression of one side of a hoof. He lowered his chin and stared at the traces as though he would force them to speak to him. Then dog-like, uncomfortably on all fours yet unheeding his discomfort, he stole forward five yards and stopped.