YES(?, 2*c + 7)

Initial complexity problem:
1:	T:
		(1, 1)    f0(a, b, c, d) -> f6(0, 0, c, d)
		(?, 1)    f6(a, b, c, d) -> f6(a, b + 1, c, d) [ c >= b + 1 ]
		(?, 1)    f6(a, b, c, d) -> f6(a + 2, b + 1, c, d) [ c >= b + 1 ]
		(?, 1)    f15(a, b, c, d) -> f19(c + 1, b, c, 1) [ a = c + 1 ]
		(?, 1)    f15(a, b, c, d) -> f19(a, b, c, 0) [ c >= a ]
		(?, 1)    f15(a, b, c, d) -> f19(a, b, c, 0) [ a >= c + 2 ]
		(?, 1)    f6(a, b, c, d) -> f15(a, b, c, d) [ b >= c /\ c >= a + 1 ]
		(?, 1)    f6(a, b, c, d) -> f15(a, b, c, d) [ a >= c + 1 /\ b >= c ]
		(?, 1)    f6(a, b, c, d) -> f19(a, b, a, 1) [ b >= c /\ a = c ]
	start location:	f0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(1, 1)    f0(a, b, c, d) -> f6(0, 0, c, d)
		(?, 1)    f6(a, b, c, d) -> f6(a, b + 1, c, d) [ c >= b + 1 ]
		(?, 1)    f6(a, b, c, d) -> f6(a + 2, b + 1, c, d) [ c >= b + 1 ]
	start location:	f0
	leaf cost:	6

A polynomial rank function with
	Pol(f0) = V_3
	Pol(f6) = -V_2 + V_3
orients all transitions weakly and the transitions
	f6(a, b, c, d) -> f6(a + 2, b + 1, c, d) [ c >= b + 1 ]
	f6(a, b, c, d) -> f6(a, b + 1, c, d) [ c >= b + 1 ]
strictly and produces the following problem:
3:	T:
		(1, 1)    f0(a, b, c, d) -> f6(0, 0, c, d)
		(c, 1)    f6(a, b, c, d) -> f6(a, b + 1, c, d) [ c >= b + 1 ]
		(c, 1)    f6(a, b, c, d) -> f6(a + 2, b + 1, c, d) [ c >= b + 1 ]
	start location:	f0
	leaf cost:	6

Complexity upper bound 2*c + 7

Time: 0.177 sec (SMT: 0.169 sec)