I recently got the V2, V4, and Xulin Pen 3. The V2 and V4 are high quality and accurate. They feature a tip that hides the needle, if that appeals to you. The main difference between them is that the V4 is slightly longer than the V2.
The Xulin Pen 3 does not have a hidden-needle tip. Instead, it uses a traditional pen-style cap. It comes in a nice case with cutouts for extra needles and an additional cartridge.
The Xulin Pen 3 is very well made and is my personal favorite. I'm planning to order more Xulin Pen 3s before they sell out. Peptide Critic shipped my order fast and it was well packaged.