To see bone structure as well, run Poser, pick the T shirt by itself, and then click SETUP tab (here's where you need Pro Pack version) ... this'll show you the shirt with the bones:

I marked two of the bones with arrows cuz they are very tiny and you may overlook em ... there are bones for all the visible parts, six in all ... each bone is given the same name of the shirt part, the internal names matching, and the regular names matching ... the bones are more or less located where the parts they represent are located, starting from the abdomen and going up, and then splitting ... Now you won't have to worry too much about the general structure, in that you'll be using this structure on yer shirt, as is, rather than making one from scratch. The key'll be to get it positioned properly for yer shirt. So there'll be some minor tweaking only. You won't have to re-design it .....

Lower left red dot is the location of the ground. Shirts face opposite directions cuz the RC was rotated 180% when we were dropping cloth ... so rotate RC around 180% to face same direction as T shirt.

Actually this is a good point to import the P4 figure for a check. I moved the T-shirt over so u can see both shirts ... At this stage I still have Meshsmooth as the final modifier on the RC stack, so I can tuck here and there before the cuts. Always tweak before the Meshsmooth, and not after, otherwise u lose the smoothness. And don't meshsmooth after the cuts cuz seamlessness will be lost ...