I believe that this is really a personal decision based upon your own analysis of pros and cons. I chose to stay for a 5th year for several of the reasons you described:
1. I had an opportunity to accrue more clinical hours (and more importantly, to gain additional clinical exposure to diverse populations and settings).
2. Not only did I have an opportunity to get more papers out, but it also gave me some extra time to do additional data collection. On internship and postdoc, I wasn't starting research from scratch. Rather, I had projects to work on (and thus submit) while I also developed new ideas to pursue at my new training site.
3. I was able to defend my dissertation before leaving for internship. As psych101 said, don't underestimate the value in this. Internship is tiring, a full-time clinical job, and most likely far away from where you conducted your data collection. All of these things combined make for an unpleasant experience if you are also trying to write your dissertation.
If you want to go after year 4, you're actually completing the applications at the very beginning of that year. So really, your application and CV will only reflect 3 years' worth of graduate work. It's important to remember that, when your application is reviewed, it will be compared to those who took the extra year.
Personally, I started grad school at a young age (23), so I also felt like I had the luxury of taking the extra year. But I can also relate to the fact that others just "want to get on with their lives," as I often felt that way myself. Taking the extra year definitely prolongs the entire process of hoop-jumping that we all go through. But for me, the extra year was entirely worth it. I got interviews at all 12 internship sites that I applied to, and matched to my first choice. I don't regret my decision at all.
Good luck!