I would recommend 4 unless you have prior knowledge of an incompatible gem you can't live without. The benefits are worth it. I've built two small-scale Rails 4

I would recommend 4 unless you have prior knowledge of an incompatible gem you can't live without. The benefits are worth it. I've built two small-scale Rails 4 apps now and all incompatible gems have been easily replaced with either custom code or other gems.