For the first +:
(+ 4 5)
(inc (+ (dec 4) 5))
(inc (+ 3 5))
(inc (inc (+ (dec 3) 5)))
..
(inc (inc (inc (+ 1 5))))
..
(inc (inc (inc (inc (+ 0 5)))))
(inc (inc (inc (inc 5))))
(inc (inc (inc 6)))
(inc (inc 7))
(inc 8)
9

For the second +:
(+ 4 5)
(+ (dec 4) (inc 5))
(+ 3 6)
(+ 2 7)
(+ 1 8)
(+ 0 9)
9

First is recursive, second is iterative.