I have been wearing the Sexi Flip every summer for the past 3 years and was so excited when I aww this version. In addition, the shoes interior tags are bothersome for the feeling of the slipper, occasionally causing itching. So happy with these! In short, the UP4 sounds great, has a longer battery life, fewer BT dropouts, has a bit more pleasing sound to me (that is entirely subjective - plenty would disagree), it is smaller and lighter, and its physical controls are both easier and more intuitive. Especially while skipping rope which was a concern. I then ordered a replacement, only one side of the headphones worked. Theyre not high-quality glass?the edges arent polished, the glass is a bit thin, and some of the glass has bubbled. I wouldnt recommend these bars. We use it in our laundry room and it makes our clothes look that are drying at a boutique PLEASE DONT MAKE THE SAME MISTAKE AS ME!