You seem to have stated the following conditions:
1a. A is always enabled.
b. B is enabled only after an A step.
c. C is enabled only after a B step.
d. D is enabled only after a C step.
2. A D step eventually occurs after every A step.
What you don't make clear is whether
3a. Only a finite number of A,B,C, and D steps may occur.
b. A finite or infinite number of those steps may occur.
c. An infinite number of those steps must occur.
Conditions 1a-d are safety conditions and should be a consequence
of the definition of the next-state action.
Conditions 1a-d imply that Condition 2 is equivalent to strong
fairness of the B, C, and D actions.
That's all you need for 3b. For 3c, you just need to add weak
fairness of A. For 3a, you need to modify 1a to allow D to disable A,
and have strong fairness on such a D action that does disable A.
All of this assumes that in 3, zero is a finite number. You'll have
to modify the solutions for 3a and 3b if you want to ensure that at
least one A step occurs.