This is not coming out properly, and the seams are opening. I am facing a lot of difficulty with this issue. Why is this happening, and how can I stop the seams from opening?
Please tell me how to fix this in CC4, and share any helpful websites or reference videos.”
Thankyou 
Cloth items have to be a solid mesh. Solidify/merge mesh, or export with perspective settings in your cloth design apps (MD, Blender, etc ). But it can be identified and taken care in CC with Weld Vertices nowadays as well (if seams are not visible in default/bind pose). A quick guide:
May I ask whether the clothing you are currently using comes from another software or from Reallusion’s content store? If it’s from our content store, please provide the specific file set.
Additionally, here is a tutorial link on seam repair for your reference, which we hope can help resolve your issue. Fixing Mesh Faces
“I create clothes in Marvelous Designer, export them, and use them in CC4. I also use clothes from Sketchfab.”
Thankyou 
Thankyou
Okay Thankyou
Thank you, 4u2ges! This is a huge help to me. I was having an issue with MetaTailor, in that it “cuts up” exported/re-imported meshes along new seams, which made it hard to further edit in CC, and would fall apart when I tried to add physics, but this takes care of it with one click! I had asked on a MetaTailor site and hadn’t gotten an answer, so I’m really glad I caught this discussion!