YES(?, a + 4)

Initial complexity problem:
1:	T:
		(?, 1)    start(a, b, c, d) -> stop(a, b, c, d) [ 1 >= a /\ b = c /\ d = a ]
		(?, 1)    start(a, b, c, d) -> lbl32(a, b, c, d - 1) [ a >= 2 /\ b = c /\ d = a ]
		(?, 1)    lbl32(a, b, c, d) -> stop(a, b, c, d) [ a >= 2 /\ d = 1 /\ b = c ]
		(?, 1)    lbl32(a, b, c, d) -> lbl32(a, b, c, d - 1) [ d >= 2 /\ d >= 1 /\ a >= d + 1 /\ b = c ]
		(1, 1)    start0(a, b, c, d) -> start(a, c, c, a)
	start location:	start0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(?, 1)    start(a, b, c, d) -> lbl32(a, b, c, d - 1) [ a >= 2 /\ b = c /\ d = a ]
		(?, 1)    lbl32(a, b, c, d) -> lbl32(a, b, c, d - 1) [ d >= 2 /\ d >= 1 /\ a >= d + 1 /\ b = c ]
		(1, 1)    start0(a, b, c, d) -> start(a, c, c, a)
	start location:	start0
	leaf cost:	2

Repeatedly propagating knowledge in problem 2 produces the following problem:
3:	T:
		(1, 1)    start(a, b, c, d) -> lbl32(a, b, c, d - 1) [ a >= 2 /\ b = c /\ d = a ]
		(?, 1)    lbl32(a, b, c, d) -> lbl32(a, b, c, d - 1) [ d >= 2 /\ d >= 1 /\ a >= d + 1 /\ b = c ]
		(1, 1)    start0(a, b, c, d) -> start(a, c, c, a)
	start location:	start0
	leaf cost:	2

A polynomial rank function with
	Pol(start) = V_4
	Pol(lbl32) = V_4
	Pol(start0) = V_1
orients all transitions weakly and the transition
	lbl32(a, b, c, d) -> lbl32(a, b, c, d - 1) [ d >= 2 /\ d >= 1 /\ a >= d + 1 /\ b = c ]
strictly and produces the following problem:
4:	T:
		(1, 1)    start(a, b, c, d) -> lbl32(a, b, c, d - 1) [ a >= 2 /\ b = c /\ d = a ]
		(a, 1)    lbl32(a, b, c, d) -> lbl32(a, b, c, d - 1) [ d >= 2 /\ d >= 1 /\ a >= d + 1 /\ b = c ]
		(1, 1)    start0(a, b, c, d) -> start(a, c, c, a)
	start location:	start0
	leaf cost:	2

Complexity upper bound a + 4

Time: 0.156 sec (SMT: 0.148 sec)