Same pose in Blender is ok, but in sims 4 deform

Hi. Vertices in this area are affected by more than 4 bones, which exceeds ts4 limit. After transferring weights, you need to always Limit total.

There are other problems, unrelated to this one:
- When choosing a reference to transfer weights, uv_1 and vertex paint, you should always go for a maxis item of similar shape, specifically length. Your garment received data from something that ends at the waistline, so everything below morphs like the waistline instead of the area of the body it covers. In cas try increasing the body mass slider or hips width and you will see it;
- the garment is not vertex painted the way something with a skirt like bottom should be. It is best to always transfer vertex paint;
- see a black shadow along the bottom of the garment? when you close holes, you create sharp edges. They need to be split to actually be shaded as such;

For the future:
- uvs made in a square uv space stretch when rectangular textures are applied. Before arranging them, you must bring them to their original size with S Y 0.5. Right now uvs are stretched in different directions;
- model in quads or retopologize the garment, it will look smoother;
- when closing holes, extrude ( once) and collapse them, then unwrap;
- split sharp edges before baking textures;
- delete all areas of the body covered with clothing carefully;
- remove doubles on the body before joining it with the garment.

Do the following:
- find a maxis item of similar shape to yours ( for example, ymBody_TowelPlush), clone it, export the blend, append it in the scene, join into one, rename to Reference;
- select your parka, separate the body from the garment;
- select your garment, transfer weights, uv_1 and vertex paint as explained below, then clean weights. In red are the most important steps, data will not transfer if you'll forget to enable them
- select the body, in edit mode select everything, remove doubles;
- join the parka back with the body;
- split edges along the borders of the garment