Similar to my previous comment, best to first check that the imported shirt OBJ correctly fits the fully zero'ed figure. Import with none of the import option boxes selected to check.

If it does, then next thing to check:-

Delete the imported shirt object.
Load the shirt figure.
Ensure that the shirt is the selected figure in the scene.
Go to the Poser menu Figure > Zero figure

Does it fit now?

If yes, then go Edit > Memorize Figure
This will remove any preset bends.

Next conform to the K4 figure.

Does it fit now?