I'm trying to figure out whether the switch from 4mm to 3mm steel is a necessity. The dry weight of SS/SH is about 300t (wikipedia), and I'm going to assume here that's with 3mm and has a payload capacity of 120t-150t. Let's say the switch reduced the weight by not 25% but 15-20% because much of the rocket stays the same. Calculating backwards that would mean the 4mm dry weight would be between 300t*100/85=352t and 375t.
Which in turn means the 4mm payload capacity would be worst case 45t and best case still about 100t to LEO.
So my take is that they don't have to make 3mm work for many mission profiles but in the long run and for mars colonization they do.