Well, I haven't been playing too long, but I don't think there's a "trend" of bass player's switching up to 4 strings. I also don't think the influence would be Thaddeus either. Maurice Fitzgerald plays a four string bass every now and then, and the brother that plays for Hezekiah Walker has always used a four string (as I recall anyways). If you could blame anyone in the gospel scene, I would blame them before Thad. (Just my opinion, I'm not trying to start a "who is better" argument...they're all wwwaaaaayyyyyy better than me lol!).
I would say that migrating to a bass with less strings forces us to learn, causing us to progress a players. I have a six and a five for that reason. I learned basics on a four, bought a five the next year, and traded my five for a six string that same year. Most of my progress on bass has been on a six string bass (and I love playing chords). When I my Squier Jazz V, I realized that a lot of what I played on the sixer could be played on the Squier, I just had to learn how to do it. I kinda feel like I have more options now...
I don't know if I would play a four at church, we don't have a set list or anything like that, and the w&p leaders might start in Ab, but the MC might sing an offering song in Db or Eb. Plus, the majority of the soloists at church sing in Db or Eb! Having that B string is great lol...