I think Autumn gives you a way out. I would have noticed that the first three boys names were biblical but if you had
Joshua, Noah, Levi, Autumn and Owen I might not have worried about it much. Most important is that you love the name.
Maybe you would feel comfortable thinking about a name that could have an equally religious meaning as a FN or MN (like you did with Autumn Grace). Suggestions could include Felix, Lucien, Lior, Valor, Merit, Prosper. If you are still worried about it there are so many biblical names out there one wouldn't necessarily consider biblical you might be able to find something you both love without realizing it.
I do think it's okay to stray from biblical names for a fourth son if you really can't find a biblical name you like, but I honestly think it would be a BETTER sibset match if you could find another biblical name that you really like.
I'm going with the vast majority (or is it unanimous?). I don't think you need another biblical name, especially since your daughter's name is Autumn. I would go with a name you love that flows with the rest of your names, but it doesn't have to be a strict theme. Owen is a classic (although it does seem to be trending) that would fit pretty well with your other classics.
That said, some great biblical names:
Jonas (is this biblical? or maybe it just sounds it because of Jonah...)
Simon (I think this is biblical? either way, it fits well with your other names)
I love the name Jonas! It is probably one of my favorite boys names of the moment. But I asked a couple of weeks ago if Jonah was too close to Joshua and Noah and everyone answered that it was. It sounds kind of like one of those celebrity mash ups that the media does when a new couple get together (ex. Bennifer and Branjelina) I just assumed that Jonas was also off the table for the same reasons. Jonas is the Greek version of Jonah (and so I do consider it to be a Biblical name). Part of the difficulty with finding a boy name is that there really isn't an "unusual" Bible name in our social circles. For example, we know two boys named Malichi, a Gideon, an Isaac, a Silas. Since both Joshua and Noah are in the top twenty (and have been for years) I really don't want to pick another top 20 boy name. Or one that is heard a lot among our friends. I love distinctive names, and I really want the name that we eventually chose to be distinctive and special.
While I do think that Owen fits stylistically with Joshua, Noah, and Levi just as well as Ezra does, I am going to vote for choosing another Biblical name, especially if you've chosen the others for reasons of faith and that faith is very important to your family. Speaking from my own personal experience, I am the only one of my siblings who doesn't have a Biblical name (in the first or the middle). So in Sunday School and family devotions, when all my siblings were being told the stories of the great Bible heroes that they were named after, I felt kind of left out. Someone did point out to me that my name (Shawn) is the Irish form of John, but that didn't help matters much because John is a man! ;)
You can definitely use a non biblical name. Ezra does sound better than Owen but partly because those are the two names you wrote. I think you should just find a name that you both like!