Okay, maybe you do get it. Your comment "SoD is not impossible in IV - though inadvisable" was confusing.
Perhaps you meant to say that stacking all of your units into a single stack of homogenous units is inadvaisable. If that is what you meant, then I agree with that.
What I don't agree with is that SODs are dead or inadvisable. Quite the opposite, they are advisable in almost all situations. I understand SOD to mean a large stack of varied units, perhaps 7-8+ varied units. They stacks are key to taking over cities and protecting your own artilery. As you identified, having more than one of these SOD is also advantageous.
For the reasons I explained in my post, in nearly every situation I would rather attack my opponent with two stacks of 7-8+ units rather than three to five smaller stacks with 3-5 units each. My two large stacks would be much more able to take advantage of terain defensive bonuses and would be much better in terms of r/p/s effect. In almost every military tech period, and in almost every situation, my two large stacks be no more vulnerable to seige than the three to five smaller stacks of 3-5 units.
So to answer the original posters question, big SOD are not very hindered by the changes to seige and are still a great strategy. The R/P/S effect and the inability to defend your own seige units makes small clusters a poor alternative.
Perhaps you meant to say that stacking all of your units into a single stack of homogenous units is inadvaisable. If that is what you meant, then I agree with that.
What I don't agree with is that SODs are dead or inadvisable. Quite the opposite, they are advisable in almost all situations. I understand SOD to mean a large stack of varied units, perhaps 7-8+ varied units. They stacks are key to taking over cities and protecting your own artilery. As you identified, having more than one of these SOD is also advantageous.
For the reasons I explained in my post, in nearly every situation I would rather attack my opponent with two stacks of 7-8+ units rather than three to five smaller stacks with 3-5 units each. My two large stacks would be much more able to take advantage of terain defensive bonuses and would be much better in terms of r/p/s effect. In almost every military tech period, and in almost every situation, my two large stacks be no more vulnerable to seige than the three to five smaller stacks of 3-5 units.
So to answer the original posters question, big SOD are not very hindered by the changes to seige and are still a great strategy. The R/P/S effect and the inability to defend your own seige units makes small clusters a poor alternative.